It's really not right to jump to conclusions immediately, and assume that your partner is cheating on you, specially if it's just your intuition that tells you something is wrong. If you are really worried about your partner having another woman/man, I think the best thing to do is to ask him/her about it, but don't sound like you're accusing him/her. Just tell him/her it's making you feel anxious, and you want to hear an honest answer from him/her.