###Be careful with the naming
Aside: you might want to say a 'separate' or 'separately provided' or 'unincluded' certificate. 'Detached' vs 'embedded' signatures in PKCS7/CMS/SMIME are a fairly important feature, and although signatures and certificates are different things this could easily get confused.
#Hexdump
Yes, you can dump a PKCS7/CMS and look at the info that identifies the signer cert, which is issuer name and serial number or (rarely, and only since rfc2630 in 1999) a key fingerprint (not cert). Here is an example der signeddata not containing the cert, but embedding (not detaching) the data (since your command implies that) and using signed attributes (which you didn't indicate one way or the other):
$ xxd se181073.p7b
0000000: 3082 0262 0609 2a86 4886 f70d 0107 02a0 0..b..*.H.......
0000010: 8202 5330 8202 4f02 0101 310b 3009 0605 ..S0..O...1.0...
0000020: 2b0e 0302 1a05 0030 1706 092a 8648 86f7 +......0...*.H..
0000030: 0d01 0701 a00a 0408 666f 6f62 6172 0d0a ........foobar..
0000040: 3182 0222 3082 021e 0201 0130 2030 1331 1.."0......0 0.1
0000050: 1130 0f06 0355 0403 0c08 7365 3138 3130 .0...U....se1810
0000060: 3733 0209 0083 2c1c 0d9b 0320 b930 0906 73....,.... .0..
0000070: 052b 0e03 021a 0500 a081 d830 1806 092a .+.........0...*
0000080: 8648 86f7 0d01 0903 310b 0609 2a86 4886 .H......1...*.H.
0000090: f70d 0107 0130 1c06 092a 8648 86f7 0d01 .....0...*.H....
00000a0: 0905 310f 170d 3138 3033 3037 3036 3033 ..1...1803070603
00000b0: 3032 5a30 2306 092a 8648 86f7 0d01 0904 02Z0#..*.H......
00000c0: 3116 0414 60e6 44a5 6cb3 048e 15e6 2d88 1...`.D.l.....-.
00000d0: e311 c28e 5a4f 6d28 3079 0609 2a86 4886 ....ZOm(0y..*.H.
00000e0: f70d 0109 0f31 6c30 6a30 0b06 0960 8648 .....1l0j0...`.H
00000f0: 0165 0304 012a 300b 0609 6086 4801 6503 .e...*0...`.H.e.
0000100: 0401 1630 0b06 0960 8648 0165 0304 0102 ...0...`.H.e....
0000110: 300a 0608 2a86 4886 f70d 0307 300e 0608 0...*.H.....0...
0000120: 2a86 4886 f70d 0302 0202 0080 300d 0608 *.H.........0...
0000130: 2a86 4886 f70d 0302 0201 4030 0706 052b *.H.......@0...+
0000140: 0e03 0207 300d 0608 2a86 4886 f70d 0302 ....0...*.H.....
0000150: 0201 2830 0d06 092a 8648 86f7 0d01 0101 ..(0...*.H......
0000160: 0500 0482 0100 0eea c31c bebb b64f 5c55 .............O\U
0000170: d5a5 5202 d59e 4742 57df cbce 42c9 f6b9 ..R...GBW...B...
0000180: 954a f6dd 1336 a99a be66 2513 bbba e176 .J...6...f%....v
0000190: acbd 2cf6 6988 847f 5fcd 6f28 e23f 1097 ..,.i..._.o(.?..
00001a0: 76a7 5f65 c028 25e2 c26d e054 d3a5 d7dc v._e.(%..m.T....
00001b0: 5168 a71d 1860 b4e8 96fc e553 032d 3a03 Qh...`.....S.-:.
00001c0: 25cd 3761 3076 99a6 bf79 43ea 43a5 7438 %.7a0v...yC.C.t8
00001d0: d10b e160 dd66 f592 cea1 6ab7 221b 8ec1 ...`.f....j."...
00001e0: 9921 9ae1 f739 5b61 6495 290b 1f7f dae4 .!...9[ad.).....
00001f0: 41d5 0f9b 5acf 2331 1447 5755 a063 c7de A...Z.#1.GWU.c..
0000200: 6830 ed43 875d a733 c588 370c e161 a7e8 h0.C.].3..7..a..
0000210: 005e 8afa c07b 5ade 2ffe dfb3 c0a0 1e28 .^...{Z./......(
0000220: 9726 5679 be4e d5ae 005c 8e50 ee83 13ac .&Vy.N...\.P....
0000230: 1ea0 144c ad72 df79 d60b 9f5b 4f5f 188d ...L.r.y...[O_..
0000240: f644 52cd 932b a24b b277 323c 7991 7418 .DR..+.K.w2<y.t.
0000250: 1bf8 c997 fd2f 1652 7658 ca42 be2f dc92 ...../.RvX.B./..
0000260: 678d 8dbd 756a g...uj
#asn1parse
Now use asn1parse to dump the ASN.1 structure and most contents:
$ openssl asn1parse -in se181073.p7b -inform der -i
0:d=0 hl=4 l= 610 cons: SEQUENCE
4:d=1 hl=2 l= 9 prim: OBJECT :pkcs7-signedData
15:d=1 hl=4 l= 595 cons: cont [ 0 ]
19:d=2 hl=4 l= 591 cons: SEQUENCE
23:d=3 hl=2 l= 1 prim: INTEGER :01
26:d=3 hl=2 l= 11 cons: SET
28:d=4 hl=2 l= 9 cons: SEQUENCE
30:d=5 hl=2 l= 5 prim: OBJECT :sha1
37:d=5 hl=2 l= 0 prim: NULL
39:d=3 hl=2 l= 23 cons: SEQUENCE
41:d=4 hl=2 l= 9 prim: OBJECT :pkcs7-data
52:d=4 hl=2 l= 10 cons: cont [ 0 ]
54:d=5 hl=2 l= 8 prim: OCTET STRING :foobar
64:d=3 hl=4 l= 546 cons: SET
68:d=4 hl=4 l= 542 cons: SEQUENCE
72:d=5 hl=2 l= 1 prim: INTEGER :01
75:d=5 hl=2 l= 32 cons: SEQUENCE
77:d=6 hl=2 l= 19 cons: SEQUENCE
79:d=7 hl=2 l= 17 cons: SET
81:d=8 hl=2 l= 15 cons: SEQUENCE
83:d=9 hl=2 l= 3 prim: OBJECT :commonName
88:d=9 hl=2 l= 8 prim: UTF8STRING :se181073
98:d=6 hl=2 l= 9 prim: INTEGER :832C1C0D9B0320B9
109:d=5 hl=2 l= 9 cons: SEQUENCE
111:d=6 hl=2 l= 5 prim: OBJECT :sha1
118:d=6 hl=2 l= 0 prim: NULL
120:d=5 hl=3 l= 216 cons: cont [ 0 ]
123:d=6 hl=2 l= 24 cons: SEQUENCE
125:d=7 hl=2 l= 9 prim: OBJECT :contentType
136:d=7 hl=2 l= 11 cons: SET
138:d=8 hl=2 l= 9 prim: OBJECT :pkcs7-data
149:d=6 hl=2 l= 28 cons: SEQUENCE
151:d=7 hl=2 l= 9 prim: OBJECT :signingTime
162:d=7 hl=2 l= 15 cons: SET
164:d=8 hl=2 l= 13 prim: UTCTIME :180307060302Z
179:d=6 hl=2 l= 35 cons: SEQUENCE
181:d=7 hl=2 l= 9 prim: OBJECT :messageDigest
192:d=7 hl=2 l= 22 cons: SET
194:d=8 hl=2 l= 20 prim: OCTET STRING [HEX DUMP]:60E644A56CB3048E15E62D88E311C28E5A4F6D28
216:d=6 hl=2 l= 121 cons: SEQUENCE
218:d=7 hl=2 l= 9 prim: OBJECT :S/MIME Capabilities
229:d=7 hl=2 l= 108 cons: SET
231:d=8 hl=2 l= 106 cons: SEQUENCE
233:d=9 hl=2 l= 11 cons: SEQUENCE
235:d=10 hl=2 l= 9 prim: OBJECT :aes-256-cbc
246:d=9 hl=2 l= 11 cons: SEQUENCE
248:d=10 hl=2 l= 9 prim: OBJECT :aes-192-cbc
259:d=9 hl=2 l= 11 cons: SEQUENCE
261:d=10 hl=2 l= 9 prim: OBJECT :aes-128-cbc
272:d=9 hl=2 l= 10 cons: SEQUENCE
274:d=10 hl=2 l= 8 prim: OBJECT :des-ede3-cbc
284:d=9 hl=2 l= 14 cons: SEQUENCE
286:d=10 hl=2 l= 8 prim: OBJECT :rc2-cbc
296:d=10 hl=2 l= 2 prim: INTEGER :80
300:d=9 hl=2 l= 13 cons: SEQUENCE
302:d=10 hl=2 l= 8 prim: OBJECT :rc2-cbc
312:d=10 hl=2 l= 1 prim: INTEGER :40
315:d=9 hl=2 l= 7 cons: SEQUENCE
317:d=10 hl=2 l= 5 prim: OBJECT :des-cbc
324:d=9 hl=2 l= 13 cons: SEQUENCE
326:d=10 hl=2 l= 8 prim: OBJECT :rc2-cbc
336:d=10 hl=2 l= 1 prim: INTEGER :28
339:d=5 hl=2 l= 13 cons: SEQUENCE
341:d=6 hl=2 l= 9 prim: OBJECT :rsaEncryption
352:d=6 hl=2 l= 0 prim: NULL
354:d=5 hl=4 l= 256 prim: OCTET STRING [HEX DUMP]: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
$ # -i causes the output 'details' to be indented according to the structure;
$ # you can omit this and just use the d=depth values but that's slightly harder
#SignerInfo structure
Compare this to the structures specified by PKCS7 or its newer and more convenient IETFized form Cryptographic Message Syntax CMS rfc2315 section 9 or rfc2360 section 5.1 or rfc3369 ditto or rfc3852 ditto. First there is an outer wrapper (defined in section 7 or 3 of the above refs) which is a SEQUENCE of an OID and a context-0 tag, which for signeddata is a SEQUENCE. At offset 23 we have the version number and at 26 the set of digest AlgIds and at 39 the EncapsulatedContentInfo containing the data. Since (all) certs were omitted, at offset 64 we have the SET of SignerInfo's, defined a few pages later in section 9.2 or 5.3:
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
SignerIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
-- SubjectKeyIdentifier is defined elsewhere as OCTET STRING
SignatureValue ::= OCTET STRING
So at offset 72 we have the version of SI, at 77 the issuer name which is a SEQUENCE of normally several SETs (in my simple example only one, CommonName) each containing one or more (here one) pair (SEQUENCE) of OID and value, and at 98 the serial number; these match (and are taken from) the issuer name and serial number in the certificate. For other signing software we could instead have a context-0 tag containing an OCTET STRING which matches (and is taken from) the SubjectKeyIdentifier extension (if present) in the certificate.
To finish, at offset 102 we have the AlgId for the digest, at 120 context-0 containing the signed attributes (previously called authenticated) which is implicit SEQUENCE of pairs (SEQUENCE) of OID and SET of value(s), and at 339 the signature itself as an AlgId and an OCTET STRING.
In case you don't know, AlgorithmIdentifier which I shorten to AlgId is defined by X.509 (and thus rfc3280 and rfc5280) as a SEQUENCE of an OID for an algorithm plus 'parameters' of varying type for that algorithm -- for the algorithm I used, RSA, the parameters are NULL. This pairing is used very widely in crypto. Note the OID assigned for rsaEncryption is used for RSA signature also, although the original 1970s concept that 'signature is encryption with the private key' proved dangerously misleading and was dropped; security.SX and crypto.SX have numerous answers and comments on this.
#Successful verification
And here is a demonstration of successful verification using that cert:
$ openssl x509 -issuer -serial -subject -in se181073.cer
issuer= /CN=se181073
serial=832C1C0D9B0320B9
subject= /CN=se181073
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
$ openssl smime -verify -in se181073.p7b -inform der -certfile se181073.cer -noverify
foobar
Verification successful
