Is it possible to test a MIDI device through its connector with a multimeter?
MIDI uses a 5 mA current loop through pins 4 and 5.
Many MIDI outputs implement this with an open-collector driver, and there is no current flowing when idle, so it is likely that you will not be able to measure any specific voltage difference between the bare pins.
The mose reliable way to measure the output signal is to connect a 470 Ω resistor between these pins, and measure the voltage drop over it (or the current through it).

While MIDI itself uses a rate of 31250 baud, it is unlikely that the keyboard is able to send messages fast enough for that to be detectable (unless it has a bulk dump function). Your best bet is to use your multimeter's Hz V function, and to whack any key as fast as possible (both key press and key release will generate a message).