If your Siri remote has enough charge left to operate, you can check the battery level by going to the Apple TV Settings app and selecting “Remotes and Devices” > Remote. Here's the visual sequence…
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I think it would have been a nice touch if Apple had included a few color-keyed LEDs on the device to indicate when the battery is low. That's because it's all too easy to be left wondering why it has stopped working, out of the blue, after a few months. If one does properly conclude the battery is dead, it's not easy to recall how many hours it takes to charge. (A full charge takes 2.5 hours.)
For reference, see: “Charge your Siri Remote or Apple TV Remote.“