Maybe when he drowned that real girl?
This question is confusing and frightening. I don't think you seriously want me to answer; you could watch the movie yourself and make a pretty good guess. Like somewhere around when he tries to communicate with a person and they run in terror.
It's possible that Frankenstein never figured it out. He could tell he was different, but he was pretty dumb. I'm talking movie Frankenstein I just realized. In the book, it's even more clear when he figures it all out, because he's quite intelligent and says many actual words.
Does it make me some sort of ass that I assumed you were talking about the movie? Is it because I assumed most people don't read things, especially if they're things that can be absorbed through the eye ball? Is eye ball one word or two? Eyeball?
Pay attention! Based on the fact that Frankenstein's monster (that's why I thought you were talking about the movie...vindicated) is made up of real boys, I'm willing to be argumentative here and claim that he never figured it out. He may have been an abomination by some standards, but just like Soylent Green (spoilers right after these parentheses) he was made of people.
Short Answer: You've seen Soylent Green by now, yes? No? Fuck.