From someone who has been there, there's no point asking your husband if he is cheating. If your husband is anything like my ex, he'll just lie and lie and lie and lie and lie.
Have you noticed any change in behaviour? Sudden guarding of his phone, wearing aftershave and hair gel to go out, suddenly arranged a night out with "Trev" (funny, he's never mentioned Trev before!)?
I found out when my intended sent two messages to me that were clearly for someone else. He still tried to lie his way out of it, but I told him to just give up lying through his teeth. There was a long silence, heavy breathing and then a confession.
In hindsight I let him get away with lying for weeks and making me think I was paranoid and crazy. I should have just given him an ultimatum when I first became suspicious.
If I were you, I'd tell your husband to come clean now or clear off.