Test for queenlessness
If there is believed to be no queen present in a hive, a test frame with eggs and small larvae but no (or few) adhering bees can be placed in the hive; It will have queencells started after a couple of days if no queen is present. If no queencells are produced, then a queen is present in the hive.
(The donor frame with no queencells can always be returned if the other colony is weak). [This is a good reason why owning two colonies is sensible]. The proviso is that if laying workers are in the hive, then they won’t rear queencells. (from: http://www.norfolkbee.co.uk/beekeepers-resource/when-will-my-new-queen-start-to-lay)