I had some robbing in a nuc the other day, so I made a 3/8" thick "U" shaped shim and covered it with window screen. I tacked it over the entrance as a robbing screen, and the robbing stopped after a day.

If you see a lot of frenzied activity, but no fighting at the hive entrance, no bees beeing dragged off or ganged up on, it's probably a new bunch of foragers orienting and not robbing.
But if you do see fighting going on it's robbing, most likely.

Here is a one minute vid of the screen with an explanation on screen use and why it works: