It would depend how much money you wanted to spend and, very importantly, how long you were prepared to take doing your measurement.
A quick back of a fag packet calculation;
Assume the candle is producing 1W of heat and it is 10km away.
You would need a large reflector to focus as much of the candle's energy as possible onto a detector - say 2m diameter.
The energy intercepted by the dish would be about 2nW (a nanoWatt is a thousand millionth of a Watt).
I saw some adverts for power meters (for measuring RF power, actually, but there can't be much difference as many of them work on the heating effect of RF). Apparently, they are available with sensitivity of 1nW.
It looks like you just about COULD do it!
Of course, you would have the problem that other things in the vicinity could be producing as much heat as the candle - the old signal to noise ratio problem. I think you might need to do it with the candle hanging from a balloon (not a hot air balloon though!!!!) at 10,000m at night. That would remove most of the stray heat sources. Damn - it wouldn't burn at that altitude. Ah well, any more questions?