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Additionally, from what I have seen, a watch's glass screen would also be much thicker than a plastic one. If the screen is very flimsy or thin, it is likely made of plastic.
(Finally, you could try to test the screen's durability - a plastic screen would break more easily than a glass one. I don't recommend this, though, because I do not want you to break your watch screen!)
If none of these tests are sufficient for you, I would consider taking it to a jeweler in your area.