I got his idea from my kid's Suzuki violin teacher:
I take a pack of Tic Tacs (yes, the refreshing mint candy) and attach it to the back of the student's left wrist with a rubber band (not too tight). If the vibrato is moving correctly, the Tic Tacs should emit a satisfying, rhythmic shake.
A little juvenile, but it seems to work, at least with the younger students.
Warning: this method will not work with a roll of Mentos
I take a pack of Tic Tacs (yes, the refreshing mint candy) and attach it to the back of the student's left wrist with a rubber band (not too tight). If the vibrato is moving correctly, the Tic Tacs should emit a satisfying, rhythmic shake.
A little juvenile, but it seems to work, at least with the younger students.
Warning: this method will not work with a roll of Mentos