Originally posted by UncleVanya Oscilloscope is best.
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To measure the trigger voltage of a flash unit, allow it to charge up off the camera. You then touch the positive (not "hot" please) lead of the voltmeter to the central contact of the flash unit's shoe, and the negative (sometimes called "ground" but it is not really ground) lead of the voltmeter to the contact spring that is in the side of the shoe. The latter is a bit hidden and can be difficult to reach, and on the camera it presses against the side of the camera hot shoe receptacle.
You should not see more than about 9 volts to be safe for a modern camera. Some older flash units had over 100 or even 200 volts - crazy. Also, the centre contact must be the positive one, although it is rare for it not to be. This web page should answer many questions : -
Photo Strobe Trigger Voltages
What I cannot advise on is the other off-centre contacts on the Nikon camera, whether they are compatible with a Pentax flash unit (perhaps others here can). These auxiliary contacts do things like tell the camera when the flash is ready, and quench the flash when enough light is received TTL. Quite likely Nikon do it differently from Pentax. One way to avoid that is to use an intermediate adaptor (it should be quite cheap) that only passes the centre and edge connections through and makes no contact with the auxiliary contacts. The flash unit will then only be working in self-auto and/or manual mode.