You can check the charge level via "Settings > Remotes & devices > Remotes > Voice Remote Pro > About" on your Roku.
If I don't want to exit what I am watching just to check the charge level while charging, I just unplug from the charge cord and observe the popup that usually (not always) appears on the upper right corner of the screen.
Roku Community Streaming Expert
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X