Most DMMs will have a hard time measuring the output voltage of a JT because of the short duration high voltage spikes. The solution is to use a capacitor to integrate the spikes over time, and a diode to keep the voltage on the capacitor. Then you can measure the ordinary DC voltage on the cap in the normal way.
With a good JT you can reach very high voltages quickly, so you should monitor the voltage and make sure you don't exceed the voltage rating of your cap. Also, use a relatively small capacitance like I do here. You can store dangerous amounts of energy even from a small battery.
Here I use the 22 microFarad, 200 Volt electrolytic cap from the innards of a CFL, along with a random rectifier diode from an old TV chassis, to make the measurement on my 10mH NE-2 JT, using the venerable Fluke 83 DMM.