get an ohm meter, find the pot center, total value of pot divided in half.
this should be where it is supposed to be unless it has a self centering pot with a detent on the shaft. if it has the self centering pot then yes you will probably have to access the controller and run a calibration mode.

you do know how to access the system dont you?
hold horn button, turn on key and cycle the horn button quickly once, its kinda tricky but if you do it right it will get you in there.
now without a manual to show you the menu structure you'll just be guessing where it is... you'll need that information to find the calibration menu if it has one.