This one's easy. If you're an elephant and you're capable of asking this question by operating some sort of computer or phone, your trunk is haunted as fuck.
Oh, wait. Trunk means other things.
Though I still think this was asked by a terrified pachyderm, let's assume someone means trunk like a place where they find their father's old baseball glove in a shitty movie.
You know your trunk is haunted if:
A) It contains an old picture of someone, but in the picture they look the same age as they do now.
B) There's an ancient book in it that exposits all the important plot connections in your story.
C) It walks.
D) When you open it, it spills water on the floor. In fact, it's full of brackish water and when you get your face close, a drowned child's hand reaches up and grasps you while a symphony plays a big scary note.
E) It's full of screams.
F) Anyone was killed anywhere near it ever. And I mean near. Like if there was an industrial accident next door, or if someone got run over by a backhoe down the street a ways.
G) You hear trunk-singing in the night.
H) At the same time, each and every night, you wake with the urge to go to the trunk.
I) When you sit on it, someone thumps from underneath, like you're being rude.
J) Every time you leave the room that houses the trunk, you're dressed like a woman. But in the creepy way, with lip-stick smeared lazily across your face.
Short Answer: You know your trunk is haunted if when you try to blow water out of it, big-eared ghosts escape instead and they go: "wooooooooooooo" instead of "Bawaarrhhraaa!"