Being Fixed

Is there anyway to find out if your husband been fixed? You see I am beginning to think my husband been keeping a secret for years now. He knew in the beginning when we met I wanted to have children. He knew I would pass him up if he was a fixed man or didn't have no interest in having anymore children. I spent a couple of years trying to get pregnant and it never happen. I had been under a lot of stress and a year became another year to another year to I finely could get around and getting tested to see if it was me not being normal cause he in his life time already had 4 children and this would be my first. He swears nothing is wrong with him because well he had 4 children in his life time and I have 0. Well I finely had myself check and everything came back normal. He had a bad wreck at one time before we met and it has been more then once he has said He don't know what happen during him being in the hospital or what it could have cut. The seat belt where it lays cut into him and they had to cut or take some intestines out but I don't think that would have any bareing on him not being able to sperm up a child. Since he brought this up twice I am beginning to wonder if he isn't hiding a vasectomy and just isn't telling me. I can't prove it and I don't like to go half cocked and start assuming things. Maybe he does have a problem. I do know our sex life is lesser and lesser then what it was when we met. Anyway is their any way I could find out he had a Vasectomy?