How to measure the flash trigger voltage
To check the trigger voltage is very easy. All you need is a digital volt meter. One terminal must be connected to the ground contact of the flash and the other to the center pin, the actual trigger signal. After that the flash must be turned on and the increasing voltage should be monitored. Once the flash is fully charged the voltage should stabilize around the final trigger voltage. The displayed value should not be higher than 12V. I don't know what is happening if it is higher and I am not willing to try at this moment, all I know that my flashes have a large variety, from 5V to 350V, so if you just pick any flash you might end up with the same variation.
Four of my flashes as examples
The lowest trigger voltage has the Osram VS280 Studio which has 5V and is safe to use. The age of this flash is unknown, but I am guessing it isn't very old.