This is what I have learned from experience.
Glass and stone are cold. Plastic is warm.
Now, obviously; there would be some exceptions like if the plastic were outside when it was quite cold while the glass was inside and warm. So; they will need to be around the same temperature.
As far as exact technique, I typically just hold the object in my hand for a few seconds (or at least have my skin against it); the plastic will warm to my hand almost immediately (and eventually you will probably sweat). However, glass will feel much colder.
I'm planning to find a link to back me up on this. However, for now I will just leave it at the fact that I have been making jewelry for a while now and when I get a rag-tag box of beads from someone, this is how I tell whether they are glass (or stone) or not.
Here is a link to an E-Bay quick guide on knowing if a bead is glass or plastic.(This will work with larger objects such as what you have as well).