MIDI is unipolar NRZ serial over twisted pair, (at 50,000 baud I think) the serial signal drives a optocoupler in the receiver so the voltage is in the 1 to 5V range somewhere and the DC impedance ballpark 200 ohms.

set your meter to AC millivolts or DC volts and look for fluctuations or connect a high-efficiency visible LED and look for blinks

MIDI is unipolar NRZ serial over twisted pair, (at 31250 baud) the serial signal drives a optocoupler in the receiver so the voltage is in the 1 to 5V range somewhere and the DC impedance ballpark 200 ohms.

set your meter to AC millivolts or DC volts and look for fluctuations or connect a high-efficiency visible LED and look for blinks