Cheating is something that is different for every person. There are the types of cheating that are mostly agreed upon across the board, yet, polyamorous or open relationships. So how do you know whether or not your partner is cheating on you?
Put on your adult pants because being childish only makes these situations worse. Don't jump to conclusions but don't second guess how you're feeling. Cheating can create some serious implications for mental health and future relationships so if your partner is cheating, deal with it right away in whichever way seems the best for you and your relationship.
The first thing you need to do is understand where you and your partner stand. Friends with benefits type of scenario and you two agreed that you wouldn't be involved with other people and you SEE/CATCH them with someone else? Cheating. The next time you see them or maybe right after you catch them, talk it out. Luckily, not too many feelings are caught up with FWB-type relationships so you'll be okay. Drop the dead weight and move on. Don't know what you guys are? Talk to them first and bring up any concerns about seeing them with another person before making it official with them.
Maybe they are seeing someone else. Dump the dead weight and move on. You'll be okay and you'll find someone who knows how not lead others on. You're in a relationship and SEE/CATCH them with someone else? If they are cheating just straight dump them. You don't need that kind of pain and bullshit. If you want to talk to them after you pamper yourself with self-love, be my guest. Not everything is how it seems, but do not let yourself fall into some snake's lies.
When in a new relationship, it is smart to talk about the uncomfortable things like getting tested for STD's and whether or not they like pineapple on their pizza. Bring up what constitutes as cheating and whether or not this relationship will be an open relationship. Communication and consent are two of the most important things in a relationship to keep it healthy.