That is the problem, i dont really know. One possible solution, but very very slow, even if it was made as an action would be "blend if sliders". I would push the dark slider to the right until i have no visible pixels left. Then i would do ctrl+click on the layer to select transparency, and do inverse selection twice to see if the selection remains In tact. If it dissapears, then there were no pixels to select. Therefore i would push the slider back through one luminosity step and do the same again by inverting the selection twice.
If the selection remains, then there is a pixel or more selected. Probably that is how i would know i got the brightest pixel or pixels. Or maybe i could push the white slider to the left and look for pixels dissapearing. Then again, click for transparency, inverse, inverse.
But that is crazy. I need a quick way of doing it; just boom and there it is - brightest pixel/s selected.
if my poor english is an obtacle to understand what i wrote here, please tell me, i would post screenshots.