A tenor riddle is:
 
Q: How can you tell when a tenor is really stupid?
A: When the other tenors notice.
 
The tenor joke was posted on the newsgroup rec.music.classical on May 5, 1993.
   
 
Wikipedia: Tenor
Tenor is a type of classical male singing voice whose vocal range is between the countertenor and baritone voice types. The tenor’s vocal range (in choral music) lies between C3, the C one octave below middle C, and A4, the A above middle C. In solo work, this range extends up to C5, or “tenor high C”. The low extreme for tenors is roughly A♭2 (two A♭s below middle C). At the highest extreme, some tenors can sing up to two Fs above middle C (F5). The tenor voice type is generally divided into the leggero tenor, lyric tenor, spinto tenor, dramatic tenor, heldentenor, and tenor buffo or spieltenor.
       
Google Groups: rec.music.classical
Musicians’ Personalities
Nancy A Howells
5/5/93
Oh, the jokes we used to tell in music school.  Mostly about singers. Like: How can you tell when a tenor is really dumb?  When the other tenors notice (ergo, tenors are dumb).
 
Google Groups: rec.humor
Canonical List Of Music Humor 2/3
Reality Terabytes
8/26/94
(...)
How can you tell when a tenor is really stupid?
When the other tenors notice.
   
Twitter
GERT‏
@Gert
#badgag How can you tell when a tenor is really stupid?  When the other tenors notice.
10:20 AM - 23 Mar 2009
   
Google Books
Sensible Nonsense.. Frank’s Comic Pack:
Humour Pack Series
By Frank Paul
BookBaby
2014
Pg. ?:
Q: How can you tell when a tenor is really stupid?A: A: When the other tenors notice.
     
YouTube
Ok, here is the joke. Q: How can you tell when a tenor is real…
botcaster inc. bot
Published on Jan 22, 2016
Q: How can you tell when a tenor is really stupid?
A: When the other tenors notice.
 
Twitter
Madeline Munoz‏
@munozatmadeline
Q: How can you tell when a tenor is really stupid? A: When the other tenors notice.
5:37 PM - 26 Mar 2017