Ok, how about making a spell that works sort of like vampiric touch. It does damage directly related to the amount of power they have, give it decent lag (a risky spell to use) but it would be a means of noticing these sorts of things. I think half the problem is that no one knows what specifically constitutes someone being weak. What they view as a skilled foe may in fact not kill that often. The guessing game is harder when you're not sure how to figure out who you are exactly supposed to be hunting. Another option is to make it sensable from blood splatters similar to how rangers can time things, it'd be neat to work in a way to "taste" the blood and discern info from that. Likes, likes?