i mean you can only guess unless you have concrete evidence... but some warning signs - emotional distance - sexually unavailable - significant changes in personality (too nice, easily angered) - disappearing for periods of time - takes showers before meeting up despite not really having a reason to - keeps lots of cash in their wallet (ideally to avoid a paper trail) - perhaps are accusatory about your fidelity
i could go on but those are a few signs to look out for. they aren't cold hard facts that a person is cheating but they are flags
I found out when I opened the PC at work and found my work mate email opened. Before closing it, I couldn't help and looked at the messages she shared with my boyfriend who also works there. I discovered interesting facts that made me end up my relationship with him. If I had been confused up to that moment, starting with that day, everything was clear for me. I am glad to have the chance of discovering what was behind me.
All this is true butt. You should know your boyfriend by now. If you feel like he isn't treating you or is doing something off. Talk to him calmly ask him for the truth is he still loves you etc just ask don't assume don't kill yourself guessing is he's cheating.
I'd say a sudden change in behavior. In my cases I've been right 2/2 times. How could I tell? All of the sudden they were so sweet, so so sweet they could only have done those things out of guilt (not that they weren't doing nice things before, but the intensity of the new behaviour was off the charts weird).
you might notice that the person will be distant... that person will be rude at you without a real reason... the person won't contact you as much as often and that person will show a certain "fear" while using his phone around you