How did the designers of Super Mario Brothers 3 expect anyone to find the first Warp Whistle?

Solution 1:

There were two hints that the warp whistle was there, and both of them were contained in letters from Princess Peach.

Between worlds 3 and 4 you get this message:

The White Block contains magic powers that will enable you to defeat your enemies.

And between worlds 6 and 7 you get this one:

I am well. Please retrieve the Magic Whistle hidden in the darkness at the end of the third world

Although it's not a map and a sign, both hints together are enough to point the way to this particular whistle at 1-3. It looks like a basic way to reward a replay as you made it through the game. There are even less clues for the other whistles; one other letter makes reference to "east of the sand dunes" for the desert world whistle, and for the first fortress whistle, there's nothing to hint at it other than Super Mario Bros hid a warp zone in exactly the same way (plus as mentioned by Ryan, it was practically advertised in a movie).

Otherwise, intentional vagueness and cryptically hidden random-access secrets (like white mushroom house, and pirate ships full of money) was a good way to sell video game guides in the early Nintendo days.