Detect if top line of a layer is empty or not?

Depending on what you are trying to do you could use Document.trim(). That would remove transparent pixels from the top of images that have them and do nothing for the images that don't have transparent pixel at the top.

You could use scriptlistener to load then save the document transparency channel to an alpha channel then select the top row and get the histogram of that alpha channel.

There are other ways as well but they use a layer object so may not work correctly if the document has more than one layer.

Or do it the easy way and use Mark's suggestion. But again it tries to copy the activeLayer. Another layer may not be transparent at the top.

// select the area you want to test for transparency

try{

    app.activeDocument.selection.copy();

}catch(e){

    // deal with transparent pixels in selection

}

There are many possibilities. E.g. this Javascript works for the active layer:

var Bds = app.activeDocument.activeLayer.bounds;
if (Bds[1] > 0) {
    alert (Bds[1] + " distance from above")
    }

I would make a selection of the area and try to copy it… PS will throw an error if it's empty…

I think their are better ways though…