My shift lever fell off on the highway, so I'm looking for a new one. I can't figure out how to tell if any of these after market levers will fit. Any tips?

I don't know what ones you're talking about but if they don't say GS500 on them they probably won't fit.  The GS is kind of an oddball shift lever.  I'm pretty sure all years of GS500 interchange so you can probably find one on ebay or in For Sale for the price of a cheeseburger.  

When I was looking pretty much all I found where stock or these
http://www.flyncycle.com/category_s/4314.htm

Are we talking about shifters or clutch levers?  I thought you meant shifter. 

Either way my answer still works...I guess.  That's what I loved about the GS.  Every part would say "fits all models, except GS500"

Nice find Pachman.  I didn't even know anybody made fancy clutch levers for these bikes.