It's getting really hard to tell, especially as the years move on, and fake penises/prosthetic/slip on/strap on dicks, are looking more believable,, and also we have CGI/AI etc.
Biggest thing is to probably find out about behind the scenes information, as to whether or not the actor himself, or crew members can confirm whether or not that was his real penis or not.
Really, all in all, if it's not a porn movie, and just a regular cinema/Hollywood film or television show etc. just automatically assume it's either a fake dick/stunt penis-dildo/or it's a CGI penis, and if you don't care either or, then the ball is in your court but yeah lol.