If the layer with the text and clipping mask has been rastersized and is not a text layer Ctrl|cmd+Click layer in layers palette loads the layers transparency as a selection and only pixels which have an opacity above 50% are outline with the marching ants. If you save the selection as a new alpha channel you can look in the channels palette to see the actuall selection. You can also use Photoshop tools and adjustment on that alpha channel like a curves or levels adjustment to turn the gray areas white then cmd+click to load the adjusted channel as a selection.
and what about the clipped layer? it wont come through onto the layer below unelss the pixels are more than 50% visible?
Pixels are selected like a feathered selection I would think if you use a filter or adjustment the selected pixels with lower then 50% opacity will be adjusted also to a lesser extent