Per Wikipedia:
A balk call requires a high degree of judgment by the umpire. Often, umpires take the intent of the pitcher and the effect of the questionable action into account when deciding whether to call a balk.
With a runner on base, it is a balk when the pitcher:
* switches his pitching stance from the windup position to the set position (or vice versa) without properly disengaging the rubber;
* when going from the stretch to the set position, fails to pitch;
* throws from the rubber to a base without stepping toward (gaining distance in the direction of) that base;
* throws from the rubber to a base where there is no runner and no possibility of a play;
* steps or feints from the rubber to first base without completing the throw;
* pitches a quick return pitch, that is, pitches with the intent to catch the batter off-guard;
* pitches or mimics a part of his pitching motion while not in contact with the rubber;
* drops the ball while on the rubber;
* after a feint or throw to a base from the rubber, fails to disengage the rubber before reengaging and pitching;
* after beginning to pitch, interrupts his pitching motion;
* begins to pitch while the catcher is out of the catcher's box when giving an intentional walk;
* while pitching, removes his pivot foot from the pitching rubber, except to pivot;
* inordinately delays the game;
* pitches while facing away from the batter;
* after bringing his hands together on the rubber, separates them except in making a pitch or a throw; or
* stands on or astride the rubber without the ball, or mimics a pitch without the ball