What was the PU doing?
If he had a hand up holding off the pitcher, then no balk should be called.
Was the ball in play?
If the last thing that happened was a foul and the batter had yet to retake the batter's box, then no balk should be called.
If the ball was in play and the umpire was not holding off the pitcher it would depend, for me, on the balk. A technical mechanical balk, such as the pitcher switching from the set to the wind-up and stepping off with the wrong foot in transition would probably not get seen clearly.
A balk on a move pick to off a runner would definitely get called.