Could this be a way to tell if you are hitting harder?

[AdamGB]

I just figured in sparring and your sparring partner says "Hey you hit hard" then I was hitting hard. Either that or seeing the way people react to being hit by you

yup, samething with me. I just tell by when i hit the opponent or whoever I'm sparring and then after they tell me that i have a powerful punch.

means nothing, 99% of guys have glass jaws... :P Maybe a bit harsh but you get my point, its down to their opinion and how well they take a punch.