Personal Experience. Both Ways

Know your partner--
1. If your partner is usually pretty easy to get ahold of: Your partner dissappears for hours. You call, they either don't answer or call back delayed
2. If your partner is usually fairly laid back: Sometimes your partner will do the opposite, because they're paranoid. They will text you to let you know what's up.

Check their phone! a cheater will delete incoming calls, messages, voicemail. A number that appears on a statement more than once, late at night, or for long call durations could be a sign of cheating

You're partner is drinking more alcohol. whether to feel less guilty, escape, or feel better about themselves. Someone's who cheating is often drunk.

Check pockets! There's things like receipts, napkins from restaurants you don't normally go to, business cards, phone numbers, club membership cards, etc

Your partner always wants to know when you will be home. They want the exact time so they can plan when they'll be home from being out with their partner in crime.

Your partner stops talking to you. There's lack of intimacy, rather time spent watching tv, sitting in silence, or not being home

Or the opposite: if your partner is usually the colder less affectionate type, they are more affectionate.

Your partner accuses you for no reason.

Your partner finds every excuse to be out of the house. They leave during fights, plan trips without you, etc