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With an Apple Watch Series 4 and up you can take an electrocardiogram, which checks your heart rhythm and alerts you if it finds any irregularities in that rhythm, such as atrial fibrillation. This is not something you'd necessarily want or need to do on a regular basis, unless you have a heart condition or are experiencing some type of symptom.
To perform an electrocardiogram with your Apple Watch, open the ECG app and tap OK. Hold your finger on the Digital Crown. The app will count down as it takes the ECG. After 30 seconds, the results pop up to display your sinus rhythm and heart rate. To ensure an accurate reading, make sure your watch is on the wrist you designated in Settings. You can also add a note in the app about any symptoms you may be experiencing.
Needless to say, if you are feeling chest pains or similar issues, don't waste time with the app; call emergency services. But in certain circumstances, the app can flag possible issues that you can convey to your doctor.