By far the easiest way to measure equine BPM is to put a coloured bandage on one of the hind legs and have someone count how many times that leg touches the ground in one minute. You can of course count for 15 seconds and times the result by four.
Remember to work your horse in before taking BPM.
You will need to ascertain a BPM for each gait. If you are training below PSG or have unequal lateral flexion, it is worth taking a BPM reading for each rein in canter to check that there is not a huge difference in speeds. If there is, take an average ... though ideally you will work on the flexion till both BPM are the same!
Equine BPM are correlated: canter is roughly twice the speed of walk, with trot mid-way between, though conformation variables and training (balance) can alter the figures.
Equine BPM (above 14:2hh) are generally between these parametres:
Walk: 50 - 65 BPM
Trot: 75 - 90 BPM
Canter: 95 - 110 BPM
Passage/Piaffe 60 - 65 BPM