The advantage of the M320 system is that it's pretty easy to use. It has a leaf-sight, which is ranged for 50, 100, 150, 200, 250, 300, and 350 meters.
As for ranging a target, though, I would advise trying to range the target using your rifle's sights before switching over to the grenade launcher unless you are danger close. Usually, that's a pretty efficient way to guesstimate how far away a target is.