Around feb DH got a job which meant he had to travel to Peterborough every other week. It was higher paid and we really needed the money and the alternative was unemployment. I had no option but to be ok with it.
Everything was goiing smoothly, he'd go away for a week, come back for a week but was acting rather "not himself" when he came back but I put it down to the huge change in his lifestyle. All the travelling and staying away from home etc.
Then one night, my sister who lives in York called me and asked why DH was in York. I said he wasn't, he was in Peterborough so she said "I'm telling you, I've just seen him in Tesco". I told her it wasn't possible and it must have been someone else so reluctantly she admitted she didn't actually speak to this man so can't be 100% but if it wasn't him, he has an identical twin 
I thought no more of it.
3 weeks later he's away again and a credit card statement came through the door. Normally I wouldn't look but gut instict told me to open it. I did. It all seemed ok apart from one transaction which was a book shop in York. He would have TOLD me if he'd made any detour to York but with what my sister said, I became suspicious that he was not being honest with me and I wanted so much to call and ask why he'd been in york but I just couldn't, I don't know why.
I let it go but when he came home I couldn't help but ask lots of questions about how things were in Peterborough. He became quite snappy and said he doesn't want to go on about work when he's at home and would I just drop it.
It had been on my mind ever since. This week he's away again, he went saturday night. This morning I received an email off my sister saying along the lines of "I know you don't want to hear this but I'd be a crap sister if I didn't tell you. I've seen (DH) again in York, it's definately him and he was holding hands with another woman." She described the woman, the clothes DH was wearing, the street, the time everything.
If I ask him outright he'll just lie and then make a better job of covering it up. How can I get solid proof? There must be a way.