I don't know about "accurately", but knowing the angular units of markings on your optical device (binocular, sniper sight, scope etc) and the size of object you are looking at (e.g. assuming an average height of a standing man) you can calculate the distance. For example, the standard unit we used in IDF (I imagine this is an international standard at least in metric system) is mils, which means that a marking 1mil long on your optics corresponds to 1meter at 1000meters distance. So, for example, if you looking at a person occupying 1mil in your scope, you can assume that he is at distance of 1800meters (assuming height of 1.8m for a man). Sometimes the inverse is what you are looking for - e.g., knowing the distance from, say, the laser range finder, you'd want to find out what you are looking at in poor visibility conditions, an animal or human, for example.
Confirming the use of mils. It's standard artillery/optic angle unit in most if not all militaries.
(v=angle in mils)
Btw. How's the Sako TRG have been faring in Afghanistan?
The rifles I have used are fixed power. However, it depends on the scope and the first focal plain.
Fixed power are the way to go IMO, In military sniping you will have a spotter with a much more powerful optic.