Generally there is no need to use a multimeter to test the board any more. If you want to test anyways, you can use a multimeter to measure the voltage between +5V pin (it’s in the top-left corner of the board and is marked on the PCB silkscreen), and ground (there is a column of GND pins close to the left edge of the board). It should be around 5V when 24VAC power is plugged in.