Possible to detect if a message is PGP encrypted/signed (ciphertext)
The only possible answer to this is to check out the OpenPGP format and see if the message adheres to that format. You may need to build in some mechanisms to filter out tiny mistakes or extensions in the message though.

As Open PGP is, well, open, there should be plenty libraries that perform this parsing for you. Library recommendations are off topic here but you should be able to find one to your liking using a search engine.