Neon transformer, so it probably has a 230V primary ( as it is in Australia) and will have an output of around 16kV. Take a length of bare steel wire and attach to the output terminals and have a grounded rod attached to the case so that each is 4mm away from the rod and at least 20mm away from the other. Power it and see if it arcs there, each side having a similar arc. Careful of the HT side, it at a minimum is incredibly painful, and just gets worse, the least you will get is a very bad point burn if you are too close, it can kill quite easily.
To check current take a cheap $5 multimeter with a 10A AC range and connect it between the output terminals and power the meter on, then turn on the NST and read the AC current through the meter. If you get it wrong bye bye meter, so that is why you use a $5 meter. Short circuit current will be in the range of 20-200mA, depending on the transformer.
Funny thing is that these draw a constant input power, as they have a ferroresonant circuit inside that provides the output. This is working as the epoxy fill is not cracked and burnt, as this is the failure mode, it lets the smoke out violently.