At forgetting about online accounts that you left active, it's pretty standard to go out of your way to get rid of any online dating apps or accounts you had before getting serious with your s.
How to tell if you're dating a cheater
You, and only you, then there's a possibility they're trying to conceal exactly how they know you.
How do you know if you're dating a cheater
If you know for a fact that your relationship with your new guy or girl didn't have an honest beginning, there's no reason for you to think that you're the special one that will finally convince your partner to be faithful til the end.
How to know if you're dating a cheater
Bad news is that the actual numbers on cheating are hard to come by, since most cheaters, malicious or accidental, are prone to denial.
Doing things like untagging themselves from all your photos or deleting your wall posts is a massive sign that they don't want at least one of their followers to know that they're taken, and at the very least, it's worth having a discussion with them about it.
Don't necessarily believe in the mantra of "once a cheater, always a cheater," but i do believe that anyone who cheatswith you will cheaton you.