Best way to detect NaN's in OpenGL shaders

I was dividing by a counter to produce an average inside of a fragment shader, and of course when the counter is zero, the resulting color value became NaN.

That's not how floats work. When you divide by zero, you get INF, not NaN. Unless the numerator is also zero, in which case you do get NaN.

In any case, GLSL offers the isinf and isnan functions, which do what they say.

Lack of isnan is problem for WebGL and Opengl ES 2.0. Polyfill which works for all GPUs I had an opportunity to try:

bool isnan( float val )
{
  return ( val < 0.0 || 0.0 < val || val == 0.0 ) ? false : true;
  // important: some nVidias failed to cope with version below.
  // Probably wrong optimization.
  /*return ( val <= 0.0 || 0.0 <= val ) ? false : true;*/
}