The simplest way is to guage your eye against something of a known heaight at a known distance, and then project it out there. Do you see a barn or outhouse? How many stories? Typical barn is 10' per floor/story plus a pitched roof of another (having climbed a 30' ladder as a kid to paint a barn, I know it won't reach the top just barely). Now mark that height at that distance on a pencil or cartridge shell held at a consistent length from your eye. The key is consistency...if you hold your arm extended once and half bent the second time the estimation will be wrong. And then once you have a good estimation, move another 20 yards away for a safety smidgeon.
Secondly, if you know where you are going, look at the area on GoogleEarth and make printouts with circles drawn around each building at 500' to reference against known landmarks.
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