uphill or downhill

NormalizedVelocity.z = 0;
Normalize(NormalizedVelocity);
float Steepness = D3DXVec3Dot (&SlidingPlane.Normal, &NormalizedVelocity);
f(Steepness>0.0f)
{
    //downhill
}    
else
{   
    //uphill
}