How can you check Battery usage on WatchOS 15.4?

After deeper research this is what is to say about your problem

Apple doesn't let you find out which apps are using the battery on your Apple Watch, but here's what you can do.

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To check your Apple Watch's battery health:

1. Press the Digital Crown on your Apple Watch.
2. Go to Settings.
3. Scroll down and select Battery.
4. Scroll down and tap Battery Health.

If this didn’t solve your problem please tell me. Thank You

Swipe up on the watch face to open Control Center and see the battery percentage. On some watch faces, you can add a battery complication. On your iPhone, you can add a battery widget to show any connected Apple Watch

To understand Apple Watch performance and its relation to your battery, see:

Apple Watch - Battery - Apple

Apple Watch Battery and Performance - Apple Support

“Battery life” is the amount of time your device runs before it needs to be recharged. “Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own. See the "Tips for Apple Watch" section in:

Batteries - Maximizing Performance - Apple

Got it sorry I didn’t understand at first here,

To access the iPhone battery level on the Apple Watch simply swipe up on the watch face. Then swipe left or right until the iPhone battery level appears on the watch.

hope this solves your problem once for all