One way to measure impact is when people come back and say things to you. And I've gotten a lot of positive feedback from people - former students who stay in touch and who tell me about their exploits in life and how they're addressing things....
That says to me that they've got something that they are carrying with them that I was able to participate in planting in them because, of course, who they are and become is largely their own doing. If they're going to take something, it's because of them, it's not because of me. I put it there, but they decide to pick it up and carry it.