Minecraft wiki entry for Defeating_the_ender_dragon, associated text: Doors flush with the wall always lead away from the entrance staircase.
Both spiral and straight (cobblestone) staircases can only lead downward from the entrance staircase.
The entrance point of a five-way crossing is always the one opposite to the arch.
The portal room is at most 5 corridors from the entrance, so if you have go to far without fiding the portal room, you are likely to be on the wrong way, so you should head back to the entrance and start discovering again.
If you heard the sound of silverfish or lava, the portal room is likely nearby.