I've performed LS fit to data in order to fit the following quadratic function:
$$f(x,y) = A~x^2 + B~y^2 + C~x~y + D~x+E~y +F$$ Now, I would like to check that the fitted function looks like a cup, or half a hose (not an inverse cup, not a saddle). is checking that $A>0$ and $B>0$ is enough?