Exterior grade plywood, paneling, etc. should be marked, such as CD-X for C grade on one side, D grade on the other, and X for exterior.
I have never seen exterior grade luan nor, as troubleseeker said, have I ever heard of it being approved for underlayment, although it is used as a spacer and leveler sometimes.
I see. I was just at lowes and saw some exterior grade lauan but it wasn't marked as far as I could tell. The only reason why I knew it was exterior grade was because that's what it said next to the price tag. They sold smaller 2x4 sheets that wasn't labeled as such but I noticed that the middle layer was much thinner than the 4x8 exterior grade lauan I saw but still 1/4". Not sure what to make of it. I realize it's not approved but many contractors and diyers do use it for that purpose.