How do you know watch is charged

Besides what Ferd II said, you can also look at the Battery Charging widget on the paired iPhone while the watch it's on the charger.

How to add and edit widgets on your iPhone - Apple Support

RossKlam wrote:

How do I know my new Apple Watch is charged

Check your battery and charge your Apple Watch provides information on how to "Check your battery at a glance from the watch face, Control Center, or Nightstand mode."