vikkii - It's always very difficult for people to learn to trust again after cheating, and that's normal. But you need to work through this issue, because not all girls cheat, and constant distrust can ruin your relationship.
Among the signs that your girlfriend is texting with other guys could be:
1. The girl deletes all messages with other guys;
2. The girl talks about other guys;
3. The girl hides her phone;
4. Leaves the room to text or talk;
5. The girl hasn't answered my messages for a long time and other things.
To understand how to know if your girlfriend is having sex or flirting with another guy online you can here https://cheatingbuster.com/how-to-tell-if-your-girlfriend-is-texting-another-guy/ , there are some methods that are sure to help you and you can figure out how to proceed.

aia - I think it's immediately obvious that the person behind your back is corresponding with another person. And it's not that hard to check, or even better to talk to your partner.