How to check battery capacity for apple watch

You should see your battery health on your watch in Settings/Battery/ ->Battery Health.

If you see your current charge level, scroll down a bit to be able to tap on Battery Health.

Ingo2711 has the answer, if you want to know the maximum capacity your battery can hold.

If you just want to know the current charge today, ”Hey Siri, battery” will tell you how much charge is left.

Control Center will also tell you the charge left for today

Use Control Center on Apple Watch - Apple Support