With the power released, if the watch is in beat the pallet fork will be centered between the banking pins on the main plate. It is usually fairly easy to see if the pallet fork is centered.

It is put in beat by turning the hairspring collett on the balance staff.

Shows watch out of beat, the pallet is not centered between the banking pins

On some watches, it is difficult to see the pallet since it is sometimes covered up by the hairspring or pallet bridge.  On this Hamilton 992b, the pallet is fairly easy to see.

Hamilton pocket watch, blue arrow shows the pallet “out of beat” not centered between banking pins
Balance wheel showing the stud and center collet. The collet needs to be rotated so that the stud moves forward or back.

You can also adjust this before putting the watch together –with only the balance installed, the jewel must sit on a line between the escape and balance jewels (and pallet pivot jewels), and if you can see through, exactly between the banking pins when looking down this line.

The stud has been adjusted and the pallet is now centered between the banding pins.

Getting the beat correct makes a large difference in the way the watch runs. It will start instantly when wound, and keep time in positions much better, have better balance rotation.

Timing machine before adjustment, watch is out of beat and running slow.
The watch is now in beat, regulated and running 6 seconds a day fast,that is about as good as it gets!!

Movable Stud

On more modern watches, they are made with an adjustable stud — quite easy to put into beat as you just move the regulator arm.

The A arrow shows the regulator, and the B shows the movable stud
This Rolex movement also has a movable stud but no regulator. A Rolex is regulated by adjusting the 4 tiny screws on the balance wheel.