There is a novel called "Point of Impact", I believe, in which the author gives some purported old hunting guides. Kind of like "if you can clearly distinguish the eyes, the distance is less than 50 yards, etc." I will try to find the book at home and post more accurate details.
Disclaimer: I have no idea if the author was writing fiction in this case, or whether there is some basis in fact. But it all sounded pretty reasonable at the time I read it.
If you have $$ to spend, Cabela's has low-tech optical rangefinders starting about $100, and hi-tech laser rangefinders for much more. If you have one of those, and time to use it, you won't have to estimate.