1x12 Speaker cab

Hey,

I have a couple of 12" speakers from an old crate 2x12 combo. I just want to take 1 and, put it in its own box to use with a Marshall DSL 2000. I know the back of the head has an output for 4+8 ohms and 16 ohms. I'm just not sure on how to wire the speaker up. There are now markings on it that would let you know how many ohms it is or which connections are the negative or positive. I'm not sure but, I'm assuming that the left connection (when looking at the back of the speaker) is the negative. How do I figure out how many ohms the speaker is? Also, when I wire the speaker to the 1/4" jack. I believe the negative is supposed to be connected to the "ring" and the positive to the "tip". Any help is appreciated.
-Matt