How do you know when a drummer is at the door?

The knocking speeds up.

How do you know when a singer is at the door?

They don't have the key, and they never know when to come in.

If we're telling mean musician jokes, my favourite:

Q. What do you call a bass player without a (girl|boy)friend?

A. Homeless.

Heh. Or, "How do you know the stage is level?"

"The drummer* drools out of both sides of his mouth."

*or bass player