Does she cheat on me

you need to go down on her for inspection...
1) Any smell of latex (condom), may not use latex type so not always accurate.
2) Taste her, is she sweet or musty (when was her last shower / wash), or salty, if cum was left behind you will find her pussy creamy smooth but tight, if open and clear no cum, but the salty / semen smell is always key.
3) If she has been active she will typically be horny if not sore (Lips will swell as well the clit if she has been active), so here is where your knowledge and history of her will be key, compare !
4) Start looking at the little details, put your hand between her legs more often and see her reaction, if she pushes away at times may give you another clue.

Check her phone texts, or your phone bill call logs for her cell...