Open up the ends and look at them. Amazing what this will reveal.
I did a festival once at the Bicentennial park in Baytown. Mix position was across a small saltwater creek from the stage. A helper managed to dip the fan of my snake in the saltwater, unknown to me. I went ahead and did the gig. The phantom power from my mixer evidently was enough to react with the salt. Inside every XLR was gunk that looked like a really bad connection on a car battery. I had to change them all out, there was no cleaning possible.