Siri remote...how do I know when it needs charging

It would still be connected to the Apple TV box through bluetooth during charging. It would continue to send its current charge level, which you can look up in Settings﹥Remotes and Devices﹥Remote﹥Battery Level﹥…%

There will be a reminder on screen (top right corner) when the charge level of the Siri Remote drops below 20%. That would be a good moment to recharge. If you postpone and forget, there will be another reminder when goes to 10%.

Please refer to Charge your Siri Remote or Apple TV Remote