Funnily, you can understand the "hardcore" factor (heh) of your enemy by watching his moves in early game creeping. Better players somehow simultaneously do the following 3 things:

1-Watch out for the last hits while not engaging combat (of course)
2-Watch out for the enemy's last hits to deny the creep.
OR
3-Watch out for the enemy's last hits to get free hits on that hero.

Usually, the third is more effective than denying. Despite that, I don't usually do it because of my laziness.