Either you are good at guessing distances, you measure different areas that you are hunting and remember the reference points, or you get a rangefinder.

I have a couple of tricks I use. I know that the front sight on my ML will cover/blot out a human head at 100 yards. So I look for another hunter and point at him and if his head is showing a little on either side of the sight then I know he is less than 100 yards away.