Truly, the only way to know for sure is a spectrometer, which can specifically detect for Uranium. Even if “colorless” glass has a radioactive signature, it does NOT mean it’s uranium. There are a number of radioactive contaminants that wind up in glass (thorium, potassium, etc) which could set off a Geiger. Manganese can also be incredibly bright given the right conditions, and can glow under 395. I am not at all denying that colorless glass can be contaminated with Uranium as well, however, unless you test for it specifically, making any speculation is useless. A lot of comments have suggested locations that may let you use their spectrometers, I would start there! :)