In the Notification screen options, you can select batteries. It will show the battery level of the device itself and all devices attached to that device (Watch, Apple Pencil, headphones, etc.)
The battery level for my BeatsX headphones was showing on my iPad notifications, but not my iPhone. Strange, then I realized I had to select "Show More" on the notification window. It was showing my iPhone and Watch, but the BeatsX was obscured until I hit Show More.