Thankfully I haven't had to worry about it too much, as ds (2.2) has only had a temp on a few occasions and I could see that he wasn't his usual self, gave him calpol, and he was quickly ok. Got one of those digital thermometers, but cannot for the life of me get an accurate reading (Ok - any reading at all) as he won't have it in mouth and hates it under arm. I see the instructions say that underarm is not very accurate anyway.
Are there better thermometers or methods? He's thrown up tonight and got a temperature, think it's going down slightly and he's fast asleep now (done calpol, not too many clothes and bit of tepid flannelling). But I'm full of admiration because I was just looking at a few recent posts and there are clearly loads of MNers who are capable of using a thermometer and I'm not!