Unfortunately, spawners can't spawn mobs on Peaceful difficulty, so in Peaceful, you can't listen to the sounds of zombies, skeletons, or spiders to find dungeons.

Do Nether Fortresses spawn in peaceful?

Difficulty has no effect on the generation of any structures, unless you count mobs like elder guardians despawning when they are generated (they will not respawn if you switch to Easy-Hard).

Do spawners appear in peaceful?

yes. spawners still spawn on peaceful but the moment the mob is spawned, it is removed from the game.