Only sure fire way to tell if someone's an optimist or pessimist is to present them with their favourite food and their least favourite food on the same plate.
They get pissed off, they're a pessimist
They get excited, that's an optimist.
[sub]The glass half full/half empty thing is bollocks. A true optimist would call it a free drink. Or is that opportunist?[/sub]
Anyway, I'm an optimist. Just. I'd call myself an opportunist more, or a situationist. I change how I can be depending on what's going on.