It sometimes helps to do the math.
Yes, it can help but not always. Go by what you can see in the film and watch the timing of it. The women witnesses said they yelled "over here" and he suddenly looks that way and waves to them. No shots yet. He goes behind the sign, reappears and the shots start just at that point.
The ladies were very constrained. They resisted trying to get the President's attention prior to here:
And waited until the last second:
And even then there's no hands in the air waving trying to get his attention.
I suppose anything is possible. They waited until the last second and only yelled, hoping they would be heard over the motorcycle acceleration.
I think he looked their way prior to the Zapruder film beginning at Z133. And that they tried to get his attention as early as when the limousine straightened out on Elm. Probably he didn't hear them and coincidentally looked their way. In their mind, their yelling caught his attention.
Not a valid vimeo URL
One of the ladies, Mary Woodward, said the first shot missed and that the Kennedys reacted to the sound of the shot by looking around (this could possibly be the rightward head turns of the Kennedys in the Z160s-Z170s). The Connallys said they also made rightward head turns in reaction to the shot, also seen occurring in the Z160s-Z170s. Nothing concrete there; just something to think about.