I've been sparring a couple of months and I still feel a little unsure of how hard I'm hitting and whether its appropriate. It helps to have a good trainer keeping things in order, but aside from that I think that feel just comes with experience. You can always ask the other guy, but personally I would never tell someone they hit me too hard.

One thing I try to keep in mind is that the goal is to touch the guy as many times as I can. That way the focus is on speed and making contact rather than power.