|A Swiss lever escapement|
It is the same with a watch except that it is now the balance wheel which has to swing an equal amount left and right from it's "in beat" position - which for a Swiss Lever escapement is when the impulse jewel, balance staff and the lever pallet arbor are in alignment with the movement at rest. Its a little more complicated for an English lever with the pallet tangential to the escape wheel.
Before the advent of electro-mechanical and now computerised escapement analysers watch makers and repairers had to adjust watches to be in beat by visual alignment and listening to the movement through a type of stethoscope, it is impressive how they achieved the accuracy that they did but some errors are fairly easy to detect by listening to the watch.
This is my Waltham 1899 Riverside Maximus from 1901, the movement is 0.3 milliseconds (mSec) out of beat which at a nominal rate of 18,000 vibrations per hour (VHP) is an error of 0.17% - that is as close to perfect as you are likely to hear from a 114 year old watch so run the video to see what it sounds like.
The next video is of a job in process and the movement is currently out of beat by 2.1 mSec or 10%, run this video and you should notice the arrhythmia.
Correcting an out of beat movement is likely to be a combination of reforming or tweaking the hairspring (in this case it needs major reshaping due to previous tampering and there is a real possibility of work hardening causing the spring to break when adjustments are made), turning the roller or preferably the hairspring collet on the balance staff to improve alignment and possibly some adjustment of the banking for fine tuning.