The simple way to take "glue" (as used on label stickers etc) off of plastic is by moistening the glue residue with Lighter Fluid (a very refined grade of kerosene).
This will not bother Acrylic, Styrene or Polycarbonate plastics, nor glass.
If in doubt, test the fluid on small part of the plastic. Look at it in 24 hours.
Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
If you scratch around a bit in one of the corners with something sharp you should be able to tell. You might also try tapping (not in the corners) with a screwdriver handle or similar. Polycarbonate will give a duller sound than glass. The sure way would be to smack it hard with a sledge hammer. If it breaks into a million pieces it's glass. If it pops out of the frame it's Lexan.