A trigger might work:
Condition:
(nearestobject [Target,"LaserTargetW"]) distance Target < 3
You might have to adjust the number depending on the size of the target object, and also I'm not 100% sure on the laser target classname, but I know there are different classnames for each of the 4 sides, the W means that the unit/vehicle with the laser designator is on West, but don't get confused by the fact that the laser dot object for West is actualy set to East side, so it appears as an enemy. So the side of the target represents the side of the user, not the side that the dot appears on.