you can use the gradient operator to identify the movement , but if the frames are very static then you can use elementary operations to identify the changing, but that also depends on the situation, is it offline operations or real time :
You can construct a loop through which you compute the Norm of the differences between each two consecutive frames :
tolerance =rand(1) ; % to identify from Tunning measurments for iter=1:N-1 Diff=norm2(Frame(i+1)-Frame(i));....
Or simply you can compute the absolute differences without a loop and sort the vector result then compute its First derivative , and take the Max index of the derivative ; that index is the separating point between the two groups .
Without mentioning that there are sophisticated ways to do the task like the " Support Vector Machine " .