If you are trying to find the entries with numerical difference being zero, you might end up with nothing because the differences may be non-zero for all entries because they are double, and assuming these 616 entries are approximation to something.
I suggest you might wanna try to set a tolerance Tol: for example
X = sin(linspace(0,2*pi,616)');
% this gives you an X being the grid approximation to sine
Y = diff(X);
Tol = 1e-2*2*pi/616;
indY = find(abs(Y) < Tol);
% the index of the difference vector that has absolute value < Tolerance
Then X(indY) and X(indY+1) will give you the entries of X that produces these less-than-tolerance differences. If the latter is bigger than the former, then X starts to increase (approximation), vice versa.