I've been knowing this guy for a while and I really like him. But he has the same last name as my brother (my dad's son). I asked him if he was related to him and he said no, but you really never know these days. So my question is do you know of a way I could find out for sure if I am related to this guy before things go any farther?

Find out who his parents are if neither one is one of your aunts or uncles then he is not a first cousin and it's all right.

If you want to make sure he is absolutely no relation just trace each of your family trees back and see if they converge at say a great grandparent. Elder relatives may come in useful here.

The only thing with that is that there are some families that have skeletons in their closets. What if no one wants to admit it or some family does not even know it and they turn out to be related. Or if the family knows about them dating. I would hope they speak up. I know I am basing my answer on what happens in soap operas but it does actually happen in real life too.