I bought a cab and need help!

If it just a regular speaker/speakers that are in the box and there is no crossover/tweeter or anthing like that you can test it at the jack, otherwise, pull out one speaker, check it and while you're in there look to make sure the others are all the same model and look at how they're wired together, staple up any lining that's falled off over the years, etc. You can figure it out from there. You'll need a multimeter to do this. One that'll work for little stuff like this can be had for just a few dollars at most stores. There are better ones obviously but a cheap one will tell you what you need to know as far as this goes. Handy tool to have around for checking speakers, testing cables, automotive stuff too. Of course you may pull the speaker and it'll have the label intact and say what it is. A screwdriver is all you need, keep your other hand cupped on the speaker side of the screwdriver so if it slips you don't damage the cone.

 

Simplest way is to use a digital multimeter set to Ohms. It's easier to just plug in a speaker cable to the cab and get measurements from there rather than fiddle with trying to get them from the jack itself.

It will read about 3/4 of the actual rating. So if it's 4 ohm cab, it will read about 3. If it's an 8 ohm cab, it will read about 6.