My company does NonDestructive testing (Radiography, Dye Penetrant, etc). If you take the part to an NDT lab it should be about $75.00 an hour if you don't have to do a shop minimum. If you want to check yourself with Dye Penetrant it is pretty easy...You can usually get the supplies at a welding supply shop (about $4 to $8 a can) or send me a PM and I will give you some mail order sources. It just takes 1 aerosol can of each: the penetrant (oil based dye), a cleaner, and a developer (provides white contrast and helps bring penetrant out of crack).
The procedure is on the back of the cans, first clean part with cleaner and lint free rag, next apply penetrant and let it "dwell" on part for about 5 minutes, Next remove excess penetrant with lint free rag soaked in cleaner (so part appears clean with no penetrant haze), last fog on developer lightly (its a lot like foot odor spray). Should be ready to view pretty quick. The larger or deeper the crack the larger the indication is. PM me with any questions Good luck
There is some fluorescent leak detection stuff from a company call Spectronics. Pour some in your engine then inspect with a black light. Not sure I would want it in my 1.8T.