uniform half4 testInputs;  // equals (-1.25, 0, 0.75, 2.25)
uniform half4 colorGreen, colorRed;

half4 main(float2 coords) {
    half4 expected = half4(0);  // derivative of a constant is zero
    return (dFdx(testInputs.x)    == expected.x     &&
            dFdx(testInputs.xy)   == expected.xy    &&
            dFdx(testInputs.xyz)  == expected.xyz   &&
            dFdx(testInputs.xyzw) == expected.xyzw  &&
            sign(dFdx(coords.xx)) == half2(1, 1)    &&
            sign(dFdx(coords.yy)) == half2(0, 0)    &&
            sign(dFdx(coords.xy)) == half2(1, 0)) ? colorGreen : colorRed;
}