Final Straw or the Straw that Broke the Camel's Back?

Is there any way I can use a lie detector test to see if my husband has been cheating on me. I am not sure where to turn for this. He has cheated many times and he is now getting some counseling and is in 12 steps sex addiction group. I found a number on his phone and when I called it it was a woman and she hung up when I started asking questions. I want to believe him but this so usually means that he has another girl. He swears he doesn’t. I don’t know what to believe. I made him get help and he seems like he is really trying but my ability to trust him is totally damaged. I believe if I could have a way to know for sure in this instance if he was lying to me again or if it was the truth then I would know if I should stay or if this was the final straw.