Although there are more proper ways to measure the magnetic force of a magnet here is one that works well when comparing two or more:
If you have a digital or old fashioned clamp ammeter that can show D.C. current, place the magnet in the 'tooth' of the clamp. You will see the highest measurement is when the poles connect around the clamp portion.
Use a little imagination to expand what can be measured. This is only useful when doing comparisons. It does not relate directly to a standard magnet rating.
The modern digital D.C. clamp meters can also be used to see variations in nearby magnetic fields.
Current does NOT have to be flowing through the center of the clamp (special note for TPU folk) ;D