Sniff test?

How often do you leave him unattended? 😅

You don’t necessarily. It’s a question of both the parameters set within the relationship and whether or not you can trust the person.

you find him naked with another woman

you don't actually but if he's hiding something picking fights than usuakky he's most likely doing something low key or dodgy

Take his phone and run away with it

Of course if your partner is no longer interested in sex with you, that's a pretty good indicator.

Polygraph?

First does he have a two bedroom place then I say yes he's probably got a roommate.

😁

It will show in the eyes. Focus on the pupil and the cornea will 'glaze'.

I'm with @Rickg - pheremones are what attract us (more than we know). You might not smell her scent on him but someone who has been sexually active recently will have a tell-tale aroma.

Knowing his day schedule

Pay attention to details

Ask him/her. Look directly in their eyes when you do.

Unless you've mentioned exclusivity ..

thats a rough existance just living eith that on your mind

They become emotionally distant

When he didn't explore with you in all aspects and enjoyed everything without residue then he will look for another! Now integrate yourself and see you will find.

you will not know.
but you will believe whatever you choose to believe

Yep

Bug them

If their penis is damp when they come home.

if you have cold sores that are unusual, or cold sore in your vagina or your pee stings, there's a chance hes sleeping around or with someone.

Put itching powder in his underwear

Smell his pee pee.