List of NSMB2 levels with Secret Exits (no spoilers for the exit, just the level they're in)

[Kenology]

Mr Khan said:
Haha, this thread is paying off already. I got all the secret exits of World 5 in my spare time. 5-1's is the hardest to find and do, requiring multiple off-paratroopa jumps.

Hints like that are good.  Don't spell anything out explicitly by tell you what you need to look for.