How can you tell how many ohms a sub has?

The best way to determine the impedance of a speaker is to look for a label or marking on the speaker. Second best is to look the speaker up in the manufacturer's specifications. Measuring a speaker with an ohmmeter does not work, because you need the average impedance at AC, not DC. You could measure the current and voltage coming from the amp at various frequencies, but you may need to compensate for phase shift, and that procedure might not agree with the manufacturer's procedure. If you are going to look up the manufacturer's procedure, you might as well look up the speaker's impedance instead.