You can check but perhaps he is just speaking to friends.
A fast way to find out is to go through his messages and call log on his phone.
How do I check to see if my husband is browsing on dating sites?
There is a free online search tool at Social Searcher that will allow you to see all web mentions, news blogs, forums and comments including Twitter, Facebook, and Whats App.
However, this site might be just as good as using Google to find out information about him.
If you can't do it because his phone is locked then leave your phone at home and make up an excuse to have to use his when you are both out somewhere. Tinder is a great one to test because it matches within proximity. Women whom he says he went to school with are strangers that he has been talking dirty to It seems you already have your answer that something untoward is happening.
Make a profile on some of the dating sites you suspect. If he is talking dirty to strangers online then he is up to something deceitful.
Let me ask you: How many times will you discover him on these sites before you allow yourself to believe that he’s basically done with his commitment to your relationship?