How to tell if you're overweight? And loose it...?

Ok I know this question has been asked before but I hate the answers because everyone says BMI but I don't think it is right. It tells me I'm obiese but I'm not! I'm 5'6. 17 yrs old, and I weigh 178. It sounds really heavy and "fat" but the thing is I'm a volleyball player, weight lifter, and I do shot put and discus, I have I lot of muscle m which makes me weigh more. Every part of me is tone except my stomach and I feel so self consience. People tell me I'm not fat, and I know the rest of me isnt but I hate my stomach, it seems like it never goes away (yet if I tighten my abs they are rock hard) I just can't seem to get the fat off over them, anyone else have this problem? How can I tell if I'm overweight w/o BMI and how do you loose stomach fat..?