How can you tell when the remote is fully charged?

There are no indicators on the remote to show it is done charging. You will need to read the charge level on the Apple TV: Settings﹥Remotes and Devices﹥Remote﹥. The icon indicates the charge level, but if you navigate one level deeper, then you’ll see a percentage as well.

Charge your Siri Remote - Apple Support

The Siri Remote takes about 2.5–3 hours to fully charge.

Settings->Remotes and Devices->Remote. Should tell you the charge level. if the battery icon is filled in, its fully charged.

hope this helps,

holyschmoe