They don't know which key to use or where to enter.
How do you know when a drummer is at your door?
The knocking speeds up.
How do you know when a trombone player is at your door?
The doorbell drags.
How do you know when a trumpet player is at your door?
The doorbell shrieks!
How do you know when there's a banjo player at your door?
His hat says "Domino's".
How can you tell that there's a vocalist at your front door?
She forgot the key and doesn't know when to come in.
How can you tell that there's an accordionist at your front door?
He doesn't stop knocking even after you answer.