It's estimated that 30-60% of all married people in the U.S. cheat on their spouse, and this number is even higher for people who are dating. This means that if you have a boyfriend or girlfriend, statistically they're more likely to cheat on you than they are to stay faithful.
Cheaters always get very protective over their phones and who can access them. Phone locks, frequent private phone calls from a "relative" they need to step away for, or changed passwords all signs they are hiding something.
There are also some cheaters who use online dating apps like Tinder, OKCupid, or one of the many other sites built for hooking up. If your partner is cheating, it's about 99% likely that they are online and chatting away on an online hookup site.