Hunted through all 20 pages of rules here, and found minimal discussion on this. So thought I'd bring it back up.
The question itself is pretty simple: When can you measure range? The specific situation I was facing last night was my opponent moved a TIE, and was considering a barrel roll. Before doing so, he pulled out the ruler to check the range and see where he would end up after the roll.
My personal read on this: The rule book lists several specific cases where you can make measurements before actually taking an action. Barrel roll, target lock, target selection… IMHO if you can measure everything at will, there's no reason for these to be listed. So even though it's not explicit, I tend to think you can't measure unless there is something that allows you to do so, such as one of the specific cases or an ability which requires a range check to work.
In general I think it hasn't been a huge deal, but I think it's about to get more important with Boost becoming a lot more common. Thoughts?