Robbing Bees
It can be difficult to identify if you have a hive that is being robbed and which hive is doing the robbing. You can dust the suspect bees with icing sugar and then see if the ghost bees are seen accessing the robbed hive. If necessary repeat daily until you find the culprits. A classic sign is bees zigzagging on their approach to the hive entrance.

If you have a strong hive they will not be troubled but nucleus colonies, especially mating nucs can be very vulnerable.

Always feed syrup late in the evening when the bees are less likely to be flying. Take every precaution not to spill syrup.

Two ways of overcoming this problem both require moving of hives. You can move the hive that is being robbed away to a new apiary for a few weeks but probably easier still is to swap the locations of the robber hive with the robbed hive. this will confuse the bees and at the same time may help to equalise the the bee populations in the two hives.

A guard bee ready to interrogate strangers.
Scientific Beekeeping
Robbing Behavior in Honey Bees, Ryan Willingham, Jeanette Klopchin, and James Ellis