Getting an accurate read on the range on beam weapons is a little tricky, because you will only get information from a hit. And if they miss because of fire control inaccuracies, you won't know what range they COULD hit at. The best way to get data is to capture examples of the ships, of course.
I build a class of scouts that are basically nothing but a boosted engine and a fuel tank, and one of their jobs is to get an estimate of the range enemy weapons have. I RP it that it is actually a sophisticated drone craft.
If you have a small stealthy scout shadowing the enemy fleet on passives, you can get an idea when they detect your fleet by when they change course in response to your fleet. I strongly recommend sending in scouts WELL ahead of your fleet to gather information. Weeks ahead, if possible. You can also do stunts like turn on a noisy active sensor on a fast scout, close in, but out of the range they could engage such a small target, and then flee and observe when they lose interest. That won't be an exact indication of the range it can detect the sensor, or if it was a planetary sensor outpost that detected the sensor or the mobile units.
If you CAN find out the exact range of the enemy missiles, then you can bleed them dry by simply having a faster ship duck into and out of that missile envelope. Carefully and tediously.