anyone who is a disc junkie has a decent chance of IDing stuff for you. Whenever my roommate and I used to lose a disc in the water and have to dig for it, anything we found we had to try to ID by feel alone before we pulled it up out of the water... we got pretty good at IDing discs based on touch. The feel of the plastic, the shape of the lip, the dome, etc...
When you add visual qualities in, too, it gets much easier.
The only problem now is that there are more manufacturers and more plastics and more molds than their used to be. I wouldn't be better than 50/50 today, because half the discs on the market are molds I haven't actually held. But still, a local pro or even an Am who loves plastic -- can probably give you the help you need.
If you don't have local pros, you can always post up pictures on this site. Take a photo of the top, bottom and the profile of the disc. Someone here will probably know what it is.