What is the test voltage of a ohmmeter?
The obvious answer is to measure it with another meter.

Other than that, this can vary by meter but is usually around a volt or two.  By default, most meters put out enough voltage to turn on ordinary silicon diodes.  Some have a special low voltage mode meant to specifically avoid turning on silicon diodes, but the accuracy is lower.

Many hand-held meters simply apply the battery voltage.  One meter I have takes a separate D cell just to power the resistance test.  It takes 4 AA cells to power the amplifier and the rest of the meter, and the single D cell provides the voltage for the resistance sense.  The one-cell voltage is about right in that it will turn on ordinary diodes, but is very unlikely to cause any harm, even of something is high enough impedance to not drag down the voltage.