how to check afib ?

Hi

Apple Watch does not currently offer AFib detection.

Under watchOS 4.x and Apple Watch Series 1 or newer, heart rate notifications can be delivered when your Apple Watch detects a heart rate that rises over the specified level:

On your iPhone, in the Watch app, go to: My Watch (tab) > Notifications > Heart Rate.

Note that these alerts are only sent when readings are detected above the specified level after you appear to have been inactive for at least 10 minutes.

• Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
• Identify your Apple Watch - Apple Support

Apple also recently announced that the Apple Heart Study app will become available in the US App Store later this year, when the first phase of the Apple Heart Study is due to begin.

The study was announced during the Apple Special Event last month, which you can view here (starting from around 24 minutes, 50 seconds):

Apple Events - Keynote September 2017 - Apple

Further information about how to participate will be provided on the following web page in due course:

http://med.stanford.edu/appleheartstudy.html

Keep in mind Apple's Important safety information, which includes:

Not a medical device Apple Watch, the heart rate sensor, and the included Apple Watch apps are not medical devices and are intended for fitness purposes only. They are not designed or intended for use in the diagnosis of disease or other conditions, or in the cure, mitigation, treatment, or prevention of disease.

Important safety information

There is no afib on the Watch yet buy there are some efforts to get it added. Use Google to find more information.