Sign 1.) You find strange things happening in your world (2 X 1 holes or 4 X 2 with redstone torches in them).
Sign 2.) You may find traps that are filled with lava, tnt, or both in many different places.
Sign 3.) You find animals in your world with white glowing eyes.
Sign 4.) You find leafless tree's in someplaces.
Sign 5) Your find your home, creations, or other buildings burned/destroyed.
Sign 6) You sometimes see him when distance is short.
Sign 7) You may find redstone Torches spawn randomly. .
Sign 8) You will also find signs by traps, most of the time saying "Hi <username>" or "Your gonna DIE".