I was in a similar situation a few months after meeting DH. he had neglected to mention that he had a child, who he saw and supported, from a previous relationship, as he thought it would "scare me off" (to be fair, I had told him I wouldn't be keen to date a guy with children- was younger and more opinionated then!)
I became suspicious, based on a few "I've got to go and help my granny move furniture" type excuses (!) Had no real evidence, but when I confronted him, I let on I knew all- he demanded to know who I had been talking to in his home town on a recent visit (actually, I found her name etc in his address book , but wasn't going to tell him that!) Every time he tried to make up a story that didn't ring true I raised my eyebrows further and said "Really? That's the version you're going to go with is it? Unless you tell me what I know (bit of exaggeration, knew nothing!) right now I will walk" He panicked, thinking I knew everything and spilled the whole story. From there we eventually managed to work it out ,although I did take a bit of a time out from the relationship to think about the implications of him having a child- I felt I had to be able to accept the fact that one day she might come to live with us, be part of our family etc, and make sure I knew I could deal with that, even if for some reason we couldn't have children. It's a big deal, no doubt about it, but we did sort it out and are now married with 3 dc. Sadly, relations between he and his ex have soured big style and we have no contact with his dd, although he still supports her financially I would actually prefer that we did have her in our lives, as I know it hurts him and hurts me to think that she might think her father doesn't want to know, but that's the way it is.
I would say that before you challenge your DH you take some time to consider about how YOU feel about this child, if it IS his. Can you welcome her into your house? Can you get by the fact that he may be a father? I know it is hard when you love someone, but if the child IS his then there is no point thinking you can turn a blind eye to her and carry on as before, I reckon.
Good luck if and when you decide to challenge him about it- challenge confidently and as if you know more than you do!