Any cracking in magnesium is usually easy to see visually due to the coarse grain structure, the same with cast alumimium. The dye checking method Michel mentioned is probably the easiest method for crack detection, but won't work if the material has been media blasted (depending on the type of media) as it moves material and can close over the cracks. Dye checking requires the item to be very clean an also paint stripped. It's easy to miss cracks when x-raying thicker material, but doesn't require paint stripping, it's time consuming and expensive. Eddy current inspection is my favourite, it's quick, easy, doesn't require paint stripping and is very sensitive, you can use different fequencies to look for cracking at different levels in the material, high freq for surface, low freq for sub surface. It requires a refence standard made of the same material as you're going to inspect with a sample crack to calibrate the machine but any NDT firm would have dosens of these. If you lived close I'd do the eddy current on them for you.
There will be NDT firms around that would do it for you if you're concerned, it's not usually cheap though. I'm lucky, I walk down to the end of our hangar and do it myself, I was in our NDT dept for three years and they still let me use the equipment. The thermal infrared is a cool toy to play with!
Alodine/Alochrome are brand names, the conversion coating is chromic acid, diluted of course, not too harmful on metal, not very friendly for your body. If you can get an alodine touch-n-prep pen use that, they're great, just apply and wait for it to dry, no need to flush the area.
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