You can show the rulers, making sure it is displaying in pixels (right-click on the ruler for the units). You can then make sure the 0,0 is in the upper left (double-click where they join).

You can always check a layer's dims by right-clicking on the image's title bar or tab, choose the appropriate action in the context menu. The X/Y will show only the current position. So if you want to display say something's X/Y at the top left, drag rulers into place.

Personally, I have never understood using PS for UI development. But it is heavily used, that's for certain. I cannot remember if Fireworks shows X/Y positions, but I used it for UIs back in the early 2000s. Before that couple year stint, I used CorelDraw for UI development, as well as afterwards. If you are subscribed, FW CS6 might be downloadable. I would check it out if so.

Mike