Use multimeter to find approximate impedance of miswired speaker array?
Place a resistor of known value in series with one speaker lead. Use a low-value resistor of sufficient wattage rating to withstand a few seconds of power. I would use a resistor somewhere between 1R0 - 3R3, 5 Watts.

Apply a tone if you have such available. 400 Hz - 1KHz is good. Increase the volume so that you can read a decent voltage across the speaker. 1 V RMS is good but less is okay if the volume is too loud.

Now simply measure the voltage across the series resistor and across the load (speakers bank). 

The ratio of the these two voltages in conjunction with the known value of the series resistor will allow you to calculate the load resistance.