View...Show...Layer Edges
If the layer's opacity in the layer window is set low, increase the opacity to 100, making note of what the original opacity was in case you want to put it back the way it was but lose the ability to 'undo'.
Command/Control click on the layer thumbnail. This will show selection bounds for anything with more than 50% opacity. If there is content, but some or all of it is less than 50% opacity, it will give you a warning message such as 'Warning: No pixels are more than 50% selected. The selection edges will not be visible.' If the layer is truly empty, it will say 'Warning: No pixels were selected'.
Edit...Free Transform. It will give you a bounding box around the content if any pixels exist on the layer. If the layer is empty it will give you a warning message such as 'Could not transform because the initial bounding rectangle is empty.'
Double click the layer to open the 'Layer Style' box. Choose 'Stroke', and slide the 'Size' slider to the right. If there is any content on the layer, you'll soon see a thick outline surrounding it.
Duplicate the layer and set the new layer's blend mode to 'dissolve'. If you see an area that had appeared empty but now is littered with dots, that's a clue that this area of the layer has pixels.

Photoshop Tutorials