I had a boyfriend a long time ago, before I was dating my husband, who was very controlling. He constantly checked to see if I was cheating in several ways. He checked through the contact list on my phone to see if I was calling any other man. He checked my text messages to see if I was texting any other man.
If I had a bruise on my neck, which I often did because I have anemia, he always accused it of being a hickey. If I wore men's antipersperant (which I have always done. it just works better.) then he thought the smell was from some other guy that I was with.
It was awful, I didn't like it, but I guess I can understand in a way. There were times when I wondered if my husband was cheating - back when we were only dating. Sometimes his clothes smelled of perfume and I wondered if I should have worried. I would smell him hard each time I hugged him.
Other than that I never really "checked" for anything. I knew he was faithful, it was just insecurity.