The key here is to find the center of gravity of your candle so that you know where you can hang it from. Here I'm holding the candle between two fingers to see if it settles correctly You can also try balancing it on a finger. Now make a mark just slightly above this point.
I'm doing this test on a new candle, but I'd recommend letting the candle burn until the flame "settles" inside the candle before doing this. The candle will be burning a little slower here and give you a bit more burn time in the end.