Spit on the head. If it sizzles and boils off almost immediately, like 1-2 secs or less, it is probably too hot. If it takes 3+ secs, you are probably ok. If it instantly vaporizes, you are way way way too hot.

Now, let me introduce you to other RC-ers who will tell you to not worry about temps. Just get the engine running perfect, then richen it slightly, make sure it sounds and looks/feels right, and forget about it. That, along with airflow past the engine, should be all you need.