A riddle about a string instrument (such as a violin or a viola) is:
 
Q: How can you tell if a violin is out of tune?
A: The bow is moving.
 
The joke has been cited in print since at least 1992.
   
“How can you tell when a politician is lying? His lips are moving!” is a related saying.
         
 
Google Groups: alt.music.filk
2 of 3: obnoxious jokes
Joe Bethancourt
4/10/92
(...)
How can you tell if a fiddle is out of tune?
The bow is moving.
 
Google Groups: rec.humor
Music Jokes
Perry Tarantine
6/12/92
(...)
How can you tell if a violin is out of tune?  The bow is moving.
 
30 June 1994, Colorado Springs (CO) Gazette-Telegraph, “This just might mean the end of the world, by Jupiter!” by Rich Tosches, pg. B1:
How can you tell when a violin is out of tune? The bow is moving.
 
Google Groups: rec.humor
Canonical List Of Music Humor 2/3
Reality Terabytes
8/26/94
(...)
How can you tell if a violin is out of tune?
The bow is moving.
   
20 August 1995, The Arizona Republic (Phoenix, AZ), “Checkered musical career not a joke,” pg. F4, col. 3:
How can you tell when a violinist is playing out of tune?
His bow is moving.
 
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 a violin is out of tune?
The bow is moving.
 
Google Books
Dave Barry in Cyberspace
By Dave Barry
New York, NY: Crown
1996
Pg. 153:
Much of the viola-joke humor appears to be based on the premise that that viola players are not the brightest or most talented members of the orchestra:
 
Q. How can you tell when a violist is playing out of tune?
A. The bow is moving.
 
A Prairie Home Companion - Jokes 1999
April 8, 1999
(...)
Q: How can you tell if a violin is out of tune?
A: The bow is moving.
Rich Pawlish, Pleasanty Unity, PA
 
Google Books
The Musician’s Joke Book
By Jim Green
London, UK: Omnibus Press
2010
Pg. ?:
Q: How can you tell if a violin is out of tune?
A: The bow is moving.
   
Google Books
Classical Music For Dummies
By David Pogue and Scott Speck
Hoboken, NJ: John Wiley & Sons, Inc.
2015
Pg. 199:
Q: How can you tell if a viola is out of tune?
A: The bow is moving.
   
Twitter
Lucky Score‏
@LuckyScoreNet
Q: How can you tell if a violin is out of
tune?
A: The bow is moving.
#Joke from #MusicJokes in #Jokes,  #Humor and #Satire
10:47 AM - 7 Jul 2017