You can know you are doing well by accomplishing the goals you set for yourself. I don't need to compare myself to another golfer to be happy I played my best game.
But surely, you set a goal with reference to something, and could that something be someone else's goal? Achieving something requires a threshold, a reference value, standard, target etc., and isn't everything we do within human interaction relative?
@Naomi actually, golf is a process and there are days when my best game is better than the worst game of the best professional golfer. However, the standards of "par" on a course are not set by the abilities of another person, but upon the complexities of the game/course. My best is better than my second best on the course - it is not related to PAR, or to what another scores on the course. It's me, against me.