Go to an audiologist. I go to one once a year and have a complete battery of tests done. The best part for me is that it is free for students here at WVU since the techs are only students. Its pretty cool, actually. They have machines that literally plot a frequency response graph of your ear. The audiology teacher was surprised that I could understand it without an explanation.
I also took steps to nip this thing in the butt by getting some of those nifty musicians earplugs for when I do live work.