When bees prepare to swarm they tend to avoid the combs. You than see the bees on the walls and in a deep floor clustering below the bottom of the frames. (Or in a fixed comb hive below the combs.) In observation hives you find many bees with their bellies turned to the window. See picture below. As said, they seem to avoid the combs.
If there are drones crowding on that bottom cluster the swarm is most likely to take of the same day.
Bellies turned to the window:
They stop any comb building. While in Spring and later in a flow they build new combs, they immediately stop to draw more combs a week or so before swarming.
A partially drawn comb (fixed comb hive, observation hive) that has been abandoned. A week before there were no bees on the window and a cluster around that comb.
With a deep floor you see something like this, a cluster below the combs:
That is "play cup", they are just playing. No real swarming attitudes.You see this by the rims of the cell/cup: the cup has a thick rim/seam.
Once the cell's rim/seam is thinned out, they are starting swarm preparations. You easily can differentiate between the thick and thin seams of the cell. If it is thinned, they start swarm preps.
Of course a capped queen cells tells you are too late. ink:
Before swarming, the bees are really really busy. Just before swarming, they stop foraging and do seem much less busy. Workers hanging around the front entrance. Combs start to shine/sparkle, because nectar is put over all over the comb, including pollen stores. If the pollen in the combs sparkles, they started swarm preparations.
Also the bees respond different on knocking the hive's wall. Knock-knock and a long muttering answer means swarm preparations. A short hiss says, they are alright. I read this in the book of Eddie Woods. See: http://beedata.com/data2/listen/listenbees.htm And I found it to be true. You don't need a gadget for this. Just your knuckles.
You find scout bees hovering around houses, sheds and buildings - those are scout bees. Once 50 or more scouts hover around and walk into a swarm trap, the swarm most likely moves in within a day. So scout bees also indicate a swarm.
Just some of the observations I made.