How do you know if your guy is faking an orgasm?

Well maybe there is a low dose, as you said.
But, a guy probably wouldn't fake it. He most likely would just keep pumping away until he gets that orgasm.

Did you ask him? Not that it is impossible but I think you'll find very few guys who feel the need to fake it.

The semen is the most obvious proof. Other proof is his face (it'll probably contort into something spectacular). The eyes are another sign, as in orgasm, the pupils dilate. Another major sign is his breathing and thrusting - they'll become more irregular and his breathing ragged.

It's pretty obvious for a guy, they cum.