There have been numerous posts asking if <insert name here> is safe to use in the GF. Good questions all, and certainly indicates that folks are thinking creatively!
Plastics come in many different formulations, often making it difficult to know whether or not a specific type contains Chlorine – a big no-no. Not only can hydrogen chloride gas do horrendous things to the optics of your shiny new GF, it can be deadly.
So, how to be certain that hunk of plastic is safe? George Graves over at Hackaday pulled together a couple of how-tos on this topic: http://hackaday.com/2015/03/14/how-to-identify-plastics-before-laser-cutting-them/ including a pretty good video of the “copper wire” test. Sure, they involve a bit of chemistry, but that’s better than taking a chance with an unknown plastic. On the other hand, fire is fun to play with!
As suggested elsewhere, Makerspaces are great resources for the budding lasercutter – if a 'space has a laser, the members have probably tested/used just about everything you’re likely to consider. In particular, the ATX Hackerspace in Austin has put together a fantastic use/don’t use list: http://atxhackerspace.org/wiki/Laser_Cutter_Materials
Have fun, but stay safe!