Replace it...
You can pressure test the tank by 1. Mop up any moisture around the tank, and around the tank fittings. 2. block the tank vent, then pumping the toilet till you feel some real resistance. 3. Go away for 24 hours. If it's leaking, you'll find moisture around it when you come back. However, I don't think you're gonna learn anything you don't already know by doing that...the tank is 10 years old, which--if it's actually been in use that long--is about double the average life of an aluminum holding tank...you've already seen blue holding tank treatment (btw, it does NOT disinfect!) in the bilge. I'd bet money it's leaking...and the first leak is ONLY the first one... it's only a matter of time--and not very much time--before more leaks start appearing. The tank is on its way to becoming a collander. Best thing to do is replace it now, at the beginning of the season, with a good quality plastic tank.