I think the man who raised me is not my dad but don't want to hurt him. How can I find out?

Talk first. As a fathers, i will be more hurt by silent suspicion than by a frank conversation. Clearly, you think this is important enough to take action. Going behind his back to plot an uncovering of what you think is family secret will create more hurt feelings in the end. It is not at all uncommon to have non-biological parent(s). Modern parents feel secure enough to tell kids the truth, either way.