Does he ask your opinion before making any plans? Does he call you if you remain absent for a long period of time( specially in case where your relationship thrives mostly on texts)? Does he make efforts to meet you quite often ? Does he include you while talking about his future? Is he overprotective of his phone while with you? Does he mind or start panicking if you scroll through his phone playfully?
These are some questions you should ask yourself. They tell a lot about the person you're with and his intentions towards you. However it's not a full proof method. Guys can cheat on you by carefully adopted techniques if they want to.