Here is a way to be able to figure out the output voltage of a microwave oven transformer. Basically we hook up the mains AC to the HV secondary of the MOT and measure the low voltage coming from the primary of the MOT. Dividing the two voltages gives us a ratio. Multiplying the line voltage by this number will give us the MOT output voltage.

Warning : The output of an MOT is lethal. Do not work on these unless you are experienced with electricity and are able to work safely with high voltage.