The quickest way is with a timegrapher and this will also give other information about the "health" of the movement on the spot, so to speak...... but will cost £150ish for a basic one if you wanted to buy one yourself....
https://vintagewatchinc.com/using-watch-timegrapher/
Alternatively, set your watch to a known good time source (eg radio controlled, GPS or internet etc) and then check again a few hours later.
https://time.is/London
If it's losing or gaining time badly then you will start to see them move out of sync within a few hours. If the difference is only a few seconds a day then you'll have to wait, well, around 24 hours or more.