Go to a welding supply store and buy a dye penetrant test kit. It has three spray cans - a cleaner, a dye and a developer.

Follow the instructions on the package:

Spray on and wipe off the cleaner.

Spray on the dye (careful - this dye is very watery and will get everywhere - do this outside, away from anything you don't want to be red), wipe it off.

Spray on the developer. Any crevice corrosion will show up as a red line.