If cracking is suspected dye pen is great but fa quick and dirty home magnaflux will tell ya what you want to know.
What you need is 2 pretty powerful magnets. Anything over 50-60 pound of force should be fine, (hobo freight, hardware store for a couple bucks to maybe 20 for the pair) something ferrous to connect them together, and some grinding dust off your shop floor. A decent sized horseshoe magnet from
Amazon is like 50 bucks that would be better but we are trying be cheap and easy to sources and a little bit sketchy here.
Clean the area as well as you can with a hand brush, you don’t have to get it shiny bright and new looking, just most of the crusty off and down to bare metal, clean well with solvent if you have oil and coolant on there
Place one magnet with the North Pole sticking. The engine block just below the freeze plug in question so your getting the bottom edge of the plug boss in between the magnets. Then place the other with the South Pole against the test part on the other side of where you suspect the flaw to be. Connect the exposed poles with a chunk of scrap steel. The magnets should only have 2-3 inches of space between them. Be careful as magnets wan to sick together more than to stuff so I would keep a small block of something between them to hold your spacing and keep your fingers from getting pinched.
Now take all the grinder dust you swept up off the floor and sprinkle it on the part between the magnets. If there is magnetic flux leakage from a flaw the ferrous metallics in the dust will stick where that flux leakage is creating a new temporary magnetic pole on the block. It’s not perfect and there is tons of field leakage with scrap steel but it should have enough field strength between the poles on the part to at least give you an idea if there is a crack there you can’t see with just a visual inspection.
I have never tried it this way as I have big old horseshoe magnet my grandpa gave that works pretty good, so your mileage may vary and may be a useless endeavor.
