It is estimated that 30-60% of married people in the U.S. cheat on their spouse, and this number is even higher for people who are dating. This means, if you have a boyfriend or girlfriend, they're more likely to cheat on you than they are to stay faithful.
One way to tell if your significant other is cheating on you is how they use their phones. Cheaters are very protective over their phones and who can have access to them. They have locks on them; frequent private phone calls from a "relative" that they need to step away for, or changing passwords frequently could mean they are hiding something.
We live in a time where anyone can cheat anytime. With social media and hookup apps, it's easy for someone who wants to cheat, to do so.