I am far from an expert, so take everything I say with a grain of salt. But my understanding is that what makes light visible in the polarimeter test is stress

I am far from an expert, so take everything I say with a grain of salt. But my understanding is that what makes light visible in the polarimeter test is stress due to tempering. E.g. my soda-lime (Pyrex) bakeware shows up very clearly in this test, but my soda-lime drinking glasses do not cause any light to appear in the test (except at their bottom). I unfortunately don't own borosilicate bakeware, so my above description of the effect comes from my linked source. The only borosilicate samples I do have are a microwave dish and a candy thermometer. The microwave dish (which has a rough texture) does show some light, but in a hazy pattern, and much less than soda-lime bakeware. The candy thermometer (which is test-tube sized) shows no light in the test. So if you're looking at bakeware, it's probable that it's either borosilicate or some other type (such as aluminosilicate used in Pyrex Flameware… again just guessing here). If you're looking at drinking glasses, then they're probably just untempered soda glass or crystalware (lead glass).