How do I know how much battery is left on my se watch

On the watch, go to Settings>Battery.

You can also set Battery as a complication on some watch faces.

M1 Mac mini, macOS 13.2.1; Watch, series 7 GPS/cellular, watchOS 9.3.1; iPad Pro 2nd Gen, iPadOS 16.3.1; iPhone 12 Pro, iOS 16.3.1; TV 4th Gen, tvOS 16.3.2; HomePods, audioOS 16.3.3 and Airport Express w/ Airplay 2

Place your finger at the bottom of the screen and drag it up to open the Control Center.

It'll show the percentage in there.

Use Control Center on Apple Watch - Apple Support