[Thrownatbirth]

Anyone know how to tell?

I just bought a set for camping. :-(

It's a clear, hard plastic that looks like glass at a distance but is lighter than you'd expect. If your first impression is "won't that crack and shatter?", it's probably Nalgene.

[varon]

"Anyone know how to tell?"

Look for this sign ;-)

...Aaaaaarrrrgh