I just found out that my dad may not be my biological father?

ever since i was younger my "dad" has been telling me things like "i've got to tell you something when you're older." i've always wondered what his Big Secret was and he always told me it was about my mom and him but he said i'd never be able to guess it and even if i did he'd say i was wrong well i talked to my mom today and finally got it out of her...apparently she doesn't know if my "dad" really is my dad or not. my "dad" has always wanted to wait until i was 18 to tell me this, but i want to find out who my real dad is! how do i bring this up to him and ask him about it? my parents are divorced and have been for a long while now so i wont have to fret about him being upset with my mom and since he was going to tell me anyways i dont see why it would matter. im currently 16 now and i just want to figure this out, any ideas and advice would be much appreciated