7. They Bring New Moves to Bed:
If they adopt a new position or kiss you in a different way, be alert. New moves can be a sign of new s.x partner in their life, although it is not necessary. They may have learned it from a friend or read about it on the internet and brought it to bed to impress you and have a good time with you.
But, on the other hand, it can also mean something else too. They may be sleeping with someone else where they used those new techniques and now confusedly brought it to you.
You can ask them where they learned it from in a way that does not imply that you doubt them. If you see them hesitating while answering it then maybe they did learn it by experimenting on another person.
All these signs are the most common ones that can help you tell if your partner is seeing or sleeping with someone else. Cheating is terrible and living with someone who is not trustworthy is not healthy, let alone acceptable.
You are not bound to spend time with someone who is cheating on you with someone else. Jerks need to be confronted and thrown out of life.