There is no range-finder in the game unless you are offline (or in TA) and are shooting at friendly tanks. In that case, you can see the icon range for a friendly, but that won't help you in the MA. There are never any enemy icons on GV's.
The key is to develop a feel for what a tank at X range will look like. To do that, try and keep a consistent level of zoom when looking at an enemy target. I know that I zoom right in to max, judge the distance, then zoom out if necessary. That way, when I'm guesstimating the range, I always know I'm looking at a target that is at the same zoom level.
-Soda
AH Trainer Corps.