It was very warm last Saturday and my 1 year old Cavalier must have been rather hot. He started to pant heavily with his tongue out, which I've not seem him do before. I've read that this can be a sign of dehydration and can be dangerous. I made sure he drank plenty of water and stayed in the shade, but he was still doing the panting and tongue hanging.

What I would like to know is if there is a way to tell the difference between just being hot and panting to cool down, and dehydration?

Try these sites, Beanie :D
Dehydration in dogs
Heatstroke in Dogs

I found out long time ago, that dogs release heat through panting and through their pads, so if it gets really hot, I'll take a damp washcloth and moisten the girls' feet.
Haven't had the trouble since we left NM, though. Just doesn't get that hot enough here in Europe to really bother them. They used to go out and sun bake when it was 100+ deg. Far. even though I'd try to keep them inside.

toodles