I have a question. If you receive a DES encrypted message for which you would NOT recognize the plaintext if it were decrypted, is there any way to potentially determine the key if all keys are likely? What about for AES?
No. For any key $k$ your ciphertext $c$ has a corresponding plaintext $p$ (since block ciphers are families of bijections). Hence, since you can't (in)validate plaintexts, you can't discard any possible key.
Or, in other words: If someone claimed the right key was a certain $k$, you had absolutely no way to ever prove them wrong.