To do this method one needs.....
1) a power amp, or receiver
2) a DMM (preferably a TrueRMS unit)
3) a method of generating low frequency sine waves (tone generators are easiest)
4) a approx 100watt, 20 ohm power resistor (there's latitude with this value, so just get close).
To test the tuning frequency of a ported sub:
Wire the resistor in series with the (+) lead going from the amp to the (+) woofer input terminal. Wire the (-) lead normally from the amp to the (-) speaker terminal.
At low volume send 50Hz sine wave into the speaker.
Set the DMM to "AC" and measure the voltage across the +/- terminals of the woofer.
Set the output from the amp so the multi-meter reads between 1&2 volts.
Now lower the frequency in steps 5Hz at a time. Each time write down the voltage reading from the meter.
As you go down in frequency there will be a point where the voltage bottoms, then starts back up again with the next lower frequency. The lowest meter reading is the port tuning frequency.
In this example the port tuning is 20Hz. That's where the meter reading shows the lowest voltage.
Obviously you can use steps smaller than 5Hz for greater accuracy.