Too hot...hmmm. Tough to describe, if stuff starts melting or it's glowing cherry red it's too hot.
Seriously though, blue flames would not be an indicator of too hot. Actually that's what you should see. The question you ask is hard to answer by desribing it but I'll give it a try.
First... coal stoves are most efficient when burned slowly. For ideal efficiency the slower the better but that's not always practical as you know since you may not got enough heat out of them.. As a general guide if you're getting a 12 hour burn time out of it then you're not running it too hot. If you were the coal would not last that long. That is what most cutomers try to achieve since it's most convenient. If you can get on a schedule like that and are getting adequate heat rthen you should be right in the "sweet spot".
Witthou using a thermometer to gauge the heat it's tough to give any other advice...