A tuning fork is often used to provide the reference pitch for tuning instruments. The tuning fork is struck and then held near the instrument being tuned. The instrument is then tuned so that it sounds in tune with the tuning fork.
Some electronic tuners can also be used to provide a reference pitch. These tuners usually have a built-in microphone that can detect the pitch of a note being played. The tuner will then display the pitch of the note and indicate whether it is in tune or not.