Forget manually taking your pulse: Your Apple Watch can tell you everything you need to know about your heartbeat.
When you start a workout, Apple Watch will begin actively monitoring your heart, recording its fluctuations every minute; after you end the workout, Apple Watch will continue this monitoring for three minutes afterward, so as to give you a "Recovery Heart Rate" (how well your nervous system regulates your heart after intense activity). You can see your heart rate at any time during your workout by raising your wrist, or by opening the Heart Rate app.
You can disable your Apple Watch's heart rate monitor altogether during workouts if you want to save power; doing so will give you a far less accurate estimate of your calories burned, though you'll still be able to record time- and distance-based metrics.
Turn on Power Saving Mode.
The Apple Watch comes equipped with a Heart Rate app that lets you manually check your heart rate at any time; in addition, it displays graphs about your resting heart rate, walking average, and any Breathe or Workout sessions you might have done.
Wait for the app to measure your heart rate.
Select the Resting Rate or Walking Average tabs in the app.
Tap on the heart rate complication anytime you want to measure your heartbeat. It will automatically open the Heart Rate app.
Apple also monitors your heart rate as you go through Breathe sessions, and adds that information to the Heart Rate app. You can check that information like so:
Scroll down to the Breathe section and tap on it.