How can you tell glass from plastic (in a non-destructive way!)?

This sounds daft, but I’ve been given a nice set of wine glasses which are made of incredibly thin glass, and tonight I bumped one against the cupboard door and to my surprise, it didn’t smash. This got me thinking, how could I tell if they actually are glass? They don’t feel like plastic, and they ‘ring’ when I tap them, but they’re much thinner than all my other glasses. Any theories anyone?