How does it look?
Is it all still nice and round throughout it's length?
If it is flattened or has noticeable kinks and dents, it will have reflections in the signal, which cause multi-path effects.
Also, be sure it does not have any holes in the outer jacket. Those would let moisture in, and likely already have.
If you're OK so far, cut off a few feet of each end. Carefully cut the outer jacket open (razor blade), and inspect the wire braid shield. There should be no corrosion, or white powdery substance in there, at all. If moisture got inside, the woven braid will wick it up... often spreading through the whole length. If it's nice and clean, you're OK.
Now, remove the braid from that section, and see if the inner insulation looks OK. If it doesn't look all cruddy, it's likely good.
Now, pull off a bit of the inner insulator, and see if the center wire looks pretty clean and un-corroded.
Basically, you need to see if water has, or can, get inside, and you need to be sure the cable is still nice and round (evenly) throughout the length that you want to use.