You feel single

Sudden unprovoked belligerence.
Visiting her 'cousin'.
When you hear "I need space".
Shopping for 10 hours.
Girls night out.

Being evasive, ie hiding parts of their life they know you know. When they become distant and cold while blaming you for their emotional ails. When they make everything about you, that's a cover up to make themselves feel better about their actions

They will accuse you of cheating.

They seem much happier

A hair clip on the floor did me in decades ago .

There is an all-pervading odour of onions in the air.

Sign! Get it?! (Jeez I'm a hoot... )

She/he becomes more protective of their phone because they have to hide texting activity. Interest in sex at home also declines.

Pick fights

My exes usually figured it out by seeing the man juice leaking down my leg

Someone else is hanging out the back of them. You wouldn’t cheat on me @SW-Questions though would you? My one true love

I would say getting caught in lies, remaining at arms length, lack of interest in sex (because they are getting it elsewhere).

They accuse you of doing such.

Sudden differences in their normal behavior, being vague or secretive...

More than 52 cards on the table.

They have sex with other people.

They don't want sex

Hickeys on her boobs were clear signs of cheating.