Grant,
I've used luan for numerous boat projects including interior veneers for a 45' catamaran, cold molded veneers for a 15' proa etc. I always used a testing method recommended by the WEST System epoxy guys ( I can't remember how to spell the name -Gougeon Brothers?).

I did the bake/boil test. Take a small sample piece and boil it in a pan on the stove for an hour. Then put it in an oven at 350 for an hour. I did this 3 times on the same sample. I found that some sheets held up and the same product bought 2 months later at the same lumber yard didn't. I don't know if it was from a different mill or what. If it failed it usually did so at the first boil. If it did hold up I went back and bought the rest of the pallet.

I've never tried to see through to locate any voids as mentioned in other posts but it seems like a good idea if it works.

Lance