Good question. There's not really a way to tell for sure - just don't take a big gulp. Blow on it a little to cool it off, and then take a sip. Like this--

[ and he demonstrates for them, gently blowing some of the steam off the top of his mug before taking an experimental sip. It's not scalding hot, fortunately, since he was trying to make it quickly - it feels nice more than anything in this cold weather. Or it does to Lupin, at least. ]

Mm. It warms you up in the winter.