Taking you own pulse (manually) is very accurate, as long as you are confident in feeling your pulse. The hardest part can be trying to coordinate counting the beats at the same time as watching a clock.

Fitbits, Apple watches, and other apps can be a good option if you find it hard to count your own pulse. Sometimes these devices can be less accurate at higher heart rates (over 120 bpm) or if you have an irregular heart rhythm.