Electromagnetic stunners damage robots, so whatever is damaged on a robot (it takes hits, but the type of damage isn't specified, but logically it must be damage to the brain since I can't see a metal or plastic limb being troubled) could be made into a target.
Realistically a simple laser sight (and a light sensor as a target) would be the easiest way of practicing. There would likely be manufacturers or third party training aids (even a laser pointer taped to the barrel would be enough).