How can I determine whether my borosilicate glass is fake or not?
Google turned up this thread: http://arstechnica.com/civis/viewtopic.php?t=96214

There don't seem to be any easy, definitive tests. You have two main options, both of which seem to require a fairly high level of laboratory skill (but nothing fancy in terms of equipment).

The refractive index of Borosilicate glass is very similar to vegetable oil (implying no reflection from a boundary if you have a pool of oil on the glass).

The density of the glass can be measured too, using a water bath. This should also be able to give a fairly accurate answer.

Stress testing seems the most reliable way to determine if you have a 'good' sample, but may be more expensive.