If , then you have a local minimum.
If , then you have a local maximum.
If for SOME h,k, and , for OTHER h,k then you have a saddle point.
You must plug in a and b. These are the points where you find stationary points to be. After that you should be able to conclude that:
is > 0 for some h k, and < 0 for other. This indicates stationary points.