I have tested my hearing since I was 16, as the years progressed, noticeable HF reduction is noticed. I used a high quality sine wave generator with a transducer of known measured response. I start at 1khz and set the volume at a moderately high, but comfortable SPL. I then vary the frequency, increasing, until I can no longer hear the signal. Not exactly the most accurate method, I admit, but it has served me for rough estimations. When I was 16, I could hear up to about 22 Khz using this method(or a little higher if I turned up the volume, thus cheating). Over a decade later, and I pooped out at about 18kHz(without cheating) when I tried this about the middle of last year.
As others have warned, the equipment must be considered. Some equipment will create lower frequency artifacts than the fundemental test tone due to various distortion(s).
-Chris