Fairly straightforward, because unlike Super Mario World, this game has no real indication of which stages have secret exits or not, and if you're like me, you don't want to spend hours hunting with a fine-toothed comb through stages for something that might not be there.

Credit to GameFaqs user Devin Morgan.

Go here if you want actual exits http://www.gamefaqs.com/3ds/667819-new-super-mario-bros-2/faqs/64824

1-Tower

2-Ghost House

3-Tower

3-4

3-Ghost House

4-Ghost House

4-Tower

5-1

5-Ghost House

5-A

6-Ghost House

Mushroom-1

Mushroom-Ghost House

Flower-1

Flower-Ghost House

If you know Spanish, I recommend you visit guiasnintendo.com, it's the official Nintendo walkthrough site managed by the Iberian division (also give it some hits for the sake of my cousin who works there :P).

For New Super Mario Bros. 2: http://www.guiasnintendo.com/index.php?option=com_wrapper&Itemid=329

Very helpful. Thank you Mr. Khan, sir.

I'm gonna need hints on a few star coins that I just can't seem to find. But first, secret exits.

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.

bookmark for later.

Got all of these, I found the 2-A exit (? block) the hardest.

It's not even a hint, really, as that's just what you have to do after you find the secret route