You see them with their pecker stuck in another bird’s nest. 😕

You can only go by gut instinct, if there's no physical evidence. I suspected my husband was, until I found a hickey on his upper thigh. Very close to his testicles. I know I didn't put it there.

Subtle or drastic changes in their behavior and/or routines. Take it from a guy who's been cheated on.

They all of a sudden refuse to have sex with you

Unexplainable itchiness.

If they seem distant and preoccupied for no apparent reason, if they have unexplained changes in mood (for the better or for the worse), if they have to work lots of overtime or suddenly have lots of places they have to go all the time, or if they suddenly become much more concerned about their physical appearance, especially after they've been involved with you for a long time and have let themselves go a bit. I think that last one is a big clue that something is going on.

If ur suspecting/questioning their faithfulness.. Unfortunately it probably means the worse...

You somehow discover it, or they tell you.

They accuse you of cheating

Smell their dick?

By being a genius.

you ask

Her pussy is way too clean.