A simple method of checking for queenlessness
Very often a colony won't give the usual signs of queenlessness, such as the "queenless roar" that appears after a few hours without a queen. There are also times when there is no brood at all in a hive and the colony is behaving as if it is queenright. Even the most experienced beekeeper can be fooled at times and get it wrong. There is no point in trying to introduce a queen if the colony is queenright, or has something that satisfies them they have a "queen", such as a laying worker, otherwise she will be rejected and a good queen may be lost.
The remedy that has been used for many years is to put a "test comb" in the suspect colony. This is simply a comb from another colony (one good reason why I advise having two colonies) that has eggs and/or young larvae that are young enough for the bees to build emergency cells on if they are queenless.
The way to do it is simple. Take a frame out of the centre of the suspect colony and shake all the bees off it. Replace it with the "test comb" that has also had all the bees removed. It is probably best if the test comb is not an old one, as young comb is preferred by bees for building emergency cells. If there is no other way of providing a queen for this colony it would be beneficial to have the test comb come from a good colony, so you have a chance of producing a reasonably good queen. It should be obvious that any combs that are transferred between colonies are free of visible signs of foul brood.
An alternative to a test comb is to give the suspect colony a sealed queen cell if you have one. If the bees are queenright they will chew it down.
In recent years I have noticed that a test comb doesn't always work, where previously it was very reliable. I believe it may be connected with some of the causes of the current queen problems.