I assume you are trying to check the length of the DH parameter for a TLS-secured service.
If you have access to the server, you can check it's configuration. It is usually configured together with the other TLS parameters of your (web?-)server.
If you don't have accees, you could use sslscan :.
> **sslscan 192.168.X.X:443**
OpenSSL 1.0.2-chacha (1.0.2g-dev)
Connected to 192.168.X.X
Supported Server Cipher(s):
Preferred TLSv1.2 256 bits ECDHE-RSA-AES256-GCM-SHA384 Curve P-384 DHE 384
Accepted TLSv1.2 256 bits DHE-RSA-AES256-GCM-SHA384 DHE 1024 bits
Accepted TLSv1.2 256 bits ECDHE-RSA-AES256-SHA384 Curve P-384 DHE 384
If you are not looking for the length, but the actual content of the parameter, you will require access to the server.