Entropy of the properly decrypted file is lower than the entropy of the improperly decoded file.
For example, properly decoded JPG file contains header information which may contain string ABCDEF which is low entropy. Also decoded email will contain header with strings (7 bit data is also low entropy).
Entropy can be measured by compressing result. Only header would be sufficient. So compressed header should be smaller than the result, which means the brute-force was successful, otherwise, the compressed result is bigger than original, which means bruteforce was unsuccessful.
Improperly decrypted file is completely random stream of bytes. Compression of it makes it bigger, but if it's correctly decrypted, should be smaller.
To make brute force faster, one can devise some quick entropy level check, however it's not feasible as of now. Currently the minimum encryption standard is AES-128 which is not bruteforcable, and AES-256 is going mainstream which has number of possibilities close to the number of atoms is observable universe. Something like some magnitudes lower than that, but very close.