Determining an Exterior Normal

Given a surface , that can be represented by the equation: $F(x,y,z)=0$ . How can I determine which of the vectors $-\text{gradF} ,\text {grad} F$ is the exterior normal and which is the interior normal?

In addition, if this surface can be represented as $z=f(x,y)$ , we know that the vectors $(f_x,f_y , -1 )$ are normals. But how can I determine who is the exterior normal and who is the interior one?

Thanks !

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I think that to make sense your question you need assume that the surface is compact! –  user52188 Jan 17 '13 at 18:38