Which terminal is the positive on Apple TV a1842 power supply?

I would use a DVM and probe the screws

The polarity is opposite to the 4th gen. Only two screws on the 5th Gen (4K) and in the first image in the post, with red rings around the terminals, the polarity is left (+) and the right (-). There are videos on how to modify the AppleTV (4th Gen) and it is a simple process. If you have a multimeter and want to verify this polarity, once the unit is disassembled, probe the terminals with the positive meter lead and connect the negative meter lead to the HDMI or Ethernet socket case. You need to have the power supply connected to the logic board to do this and powered on using the power lead. Use caution as lethal voltages exist while it is on and out of the case.