This is a very serious issue, and I have had the same questions myself. My approach is adjust the volume frequently (whenever I change programs I'm listening to, etc.), going from inaudible and increasing the volume until I can hear clearly. If you try to decrease the volume to make it a safe level from a louder level, you'll be used to having it (possibly) too loud, so you run the risk of making it less damaging, but not safe.
One thing I have noticed, is that it's very hard to tell if you are losing hearing, eyesight, or other senses. It is so gradual, it's almost impossible to detect it. You will need to see a doctor and take a hearing test for an accurate rating of your hearing. You can, however, do some of the same sorts of things at home, like finding the lowest setting you can clearly hear a specific song or speech or other media at, and revisiting it every now and again to check for discrepancies. If you notice anything changing, you may want to consult a doctor.
Hearing loss is very serious, so if you have any misgivings, consult an ear doctor.