A popular joke about drummers is:
 
Q: How do you know when the stage is level?
A: When drool comes out of both sides of the drummer’s mouth.
   
“How can you tell if the stage is level? If the banjo player drools out of both sides of his mouth” was cited in 1991. “How can you tell when the stage is level? The violists drool from both sides of their mouths” was cited in 1992.
   
“Yeah, and how do know if the stage is level? The drummer drools evenly from both sides of his mouth” was posted on the newsgroup aus.jokes on April 23, 1992.
   
 
Google Groups: rec.music.folk
by request The Canonical List of Banjo Jokes
Darrell Reich
7/5/91
(...)
How can you tell if the stage is level?
If the banjo player drools out of both sides of his mouth.
     
Google Groups: rec.music.classical
Assorted VIOLA JOKES
Nicole Marie Brockmann
3/16/92
(...)
How can you tell when the stage is level?
The violists drool from both sides of their mouths.
 
Google Groups: aus.jokes
BORED
Rowan Crowe
4/23/92
(...)
MD>Yeah, and how do know if the stage is level?
MD>The drummer drools evenly from both sides of his mouth..
 
10 May 1992, San Francisco (CA) Chronicle, “Nuclear Whales / Thar They Blow: Saxes and Slapstick / Bay ensemble walks fine line between silly and serious” by Les Hildebrand, pg. 40:
“HOW DO you tell if the stage is level?” saxophonist Rach Cztar (pronounced “rock star”) asked a Mountain View audience during a recent performance by the Nuclear Whales Saxophone Orchestra.
   
2 August 1992, Argus Leader (Sioux Falls, SD), “Sunshine, smiles go hand in hand at folk festival” by Bob Keyes, pg. F1, col. 4:
Michigan songwriter Joel Mabus humored the crowd with his jokes—“How do you know when the bluegrass stage is level? When the banjo player drools out of both sides of his mouth.”
 
Google Books
List:
Lots of List for Laughs and Lifts
By Derric Johnson
Yess Press
1993
Pg. 47:
Q. What does it mean when viola players drool out of both sides of their mouths?
A. The stage is level.
 
Google Groups: rec.music.makers.percussion
Musician Jokes!!
Michael J. Edelman
9/21/93
(...)
What does it mean when a drummer drools out of both sides of his mouth?
The stage is level.
 
Google Books
The Musician’s Joke Book:
Knowing the Score
By Nancy Groce
New York, NY: Schirmer Books
1996
Pg. ?:
How can you tell if the stage is level?
The drummer drools equally out of both sides of his mouth.
 
Google Books
The Great Rock ‘N’ Roll Joke Book
By Dave Marsh and Kathi K. Goldmark
New York, NY: St. Martin’s Griffin
1997
Pg. 8:
How can you tell when the stage is level?
The drool is coming out both sides of the drummer’s mouth.
     
Twitter
Mick_Shrimpton
‏@Mick_Shrimpton
How do you know when the stage is level?  When drool comes out of both sides of the drummer’s mouth.
12:09 PM - 24 Oct 2008
 
Google Books
Born to Drum:
The Truth About the World’s Greatest Drummers—from John Bonham and Keith Moon to SHeila E. and Dave Grohl
By Tony Barrell
New York, NY: HarperCollins
2015
Pg. ?:
“How do you know when the stage is level?” asked Kirke, quickly providing the punch line: “When the drummer drools out of both sides of his mouth.”