You can test a speakers ohm rating with a multimeter, which can be bought for about AU$20 in most electrical stores. Im not sure about the rest.

car speakers usually range at 32ohm, home audio normally 4-8 ohm, you can use pretty much any combination if ya not fussy on sound quality....for power handling wattage should match the crossover....so should the impedance but advice could go on & on....if still in doubt use matching speaker components

I would begin by trying to identify the speaker manufacturer and model number. You may be able to find the information you are looking for on the company's website or by contacting them by mail / email. If that route fails, you will need to measure the speaker's impedance. Contrary to popular belief, a speaker is not a purely resistive load and cannot be accurately measured with a simple ohmmeter or multimeter. The impedance of a speaker is a combination of resistance, capacitive reactance, and inductive reactance. Capacitive reactance is produced primarily by the capacitors in the crossover, when present. Inductive reactance is produced primarily by the inductors in the crossover, when present, and by the speaker’s voice coil, which is an inductor itself. The capacitive and inductive reactances are frequency-dependant and change with the frequency of the signal input to the speaker. Therefore, as the frequency content of the music varies, the speaker's impedance will also vary. The rated impedance is a nominal value and can vary widely across the speaker's operating range. Measuring a speaker's impedance is a moderately complex operation. Typically, the voltage divider method is used. A sine wave function generator set to a known output voltage level is connected through a power resistor to the speaker input terminals. An AC voltmeter is then placed across the speaker terminals and the voltage drop across the speaker is measured as the signal frequency is varied over the desired range (e.g., 20 Hz to 20 kHz for a full-range speaker). The speaker impedance, as a function of frequency, can be calculated from the measured voltage. Individual components should be measured over their rated operating range. A subwoofer, for example, should be tested from a frequency of 10 to 20 Hz to about 250 Hz. The power resistor could be any suitable value - something around 1000 ohms works well. It should be capable of handling 5 to 10 Watts. Note that the labelled resistance may vary by as much as 10% from the specified value. The resistance will have to be measured accurately, to within 1%. The AC voltmeter should be a 'true RMS' voltmeter, for the sake of accuracy. Step-by-step instructions detailing this process can be found in audio reference books such as: - "The Loudspeaker Design Cookbook" by Vance Dickason. - "Testing Loudspeakers" by Joseph D'Appolito.

Firstly you need to know what kind of speakers they are. A guitar or bass amp speaker is not going to sound anything like an audio speaker and vice versa. Most likely there are codes on the speakers magnet that you can search on and find out. Secondly, it depends on what you are going to use it for. If these are truly sub-woofers as you state then they will be good in a sub-woofer enclosure. They will not be good for a two 10 guitar cabinet. Lastly, as the first answer suggested, use a multimeter or take them to an electronics shop and find out the Ohm level. Then seek out some wiring diagrams for sub-woofers and look at how combining speakers changes the Ohm levels or better yet have a professional look at what you have and tell you what you need.