Unless you catch them red-handed, it's impossible to know for sure if someone's cheating. But everyone who's been in a relationship has their own theories and red flags.
A recent Reddit thread asked, "What signs do you look out for to know if your SO is cheating on you?" And the amateur detectives of the relationship world delivered.
You have those feelings for a reason. Relationships are based on gut instincts, not intellectual certainty -DeathBy1000Cuts
More distant. Less willing to talk. See less of them as time wanes - DeathBy1000Cuts
Starting a fight or making a huge drama when they arrive home so you think about that instead of where they've been. - DegeneratesInc
I think if they constantly accuse you of cheating when you've never been anything but loyal, that's a sign of their mindset, not a good one. - aflashinlifespan