I throw a couple of sucker punches to see if the guy will show me what he's got, but it depends how long the hill is. Sometimes, if you're unfamiliar with the route, you may have no idea how long the hill is, this can obviously make things tricky

If the hill is steep/slow enough to significantly reduce drafting effects, then it doesn't matter if the guy is on your wheel or not -- if you're stronger, he won't be albe to stay there.

I also like to save a little for the way down the other side, because NOBODY expects you to take off the down the other side of a hill if it's been a hard effort on the way up

It's matter of experience with your heart rate monitor, and knowing what you're capable of relative to the length/time of a particular effort.