To expand on that:MikeB wrote:Using a test meter measure the voltage on the connection on the Arduino.
Get a multimeter, preferably a digital one. Harbor Freight (a bargain tool store in the US) sells them for less than $10, so hunt around a little and you should be able to find something affordable.
Plug the black test lead into the COM socket on the meter, and the red one into the V / Ω / everything but A socket.
Set the meter to volts DC (VDC, or sometimes a solid line with dashes under it:⎓). There may also be just positions for volts (V) and a switch for DC/AC. If you have a manual-ranging multimeter, there will be several numbered positions (ranges) for volts. The number represents the maximum value the meter can read on that range setting. The lower the range, the more precise the meter is, so pick the range just above the highest voltage you expect to encounter (5V in this case). If your meter only has one setting for V or VDC, it's an auto-ranging meter which will pick an appropriate range value for you.
Open your radio and power it up. Touch the black/COM test probe to the GND pin of the Arduino, or bare metal/solder of the wire connected to the GND pin. Touch the red test probe to the (bare metal/solder of the) wire you want to test, in this case one of the encoder wires, button wires and/or encoder common wire.