I know it's not ideal and Siri should be able to tell you or something, but you can open Find My on your watch and see a battery icon on all your devices. No specific percentage though, but it might just help our lazy asses when the phone is charging aaaaall the way over there.

And on the flipside, my phone should tell me when the watch is charged!!! (it will only do that if you have 1 watch. Apple hates it when you have 2).

Annoying to have to use a paid app for this but Juice Watch can do it.