Have you ever been working on an image in PhotoShop and needed to get to the center of it? Here's a quick and easy way to do it.
Crop Tool>In Options Bar, choose "Front Image"
Drag your crop tool around entire image to select.
Command-R/(CTRL-R) to show rulers. Drag out a vertical guide from the side ruler and drop it exactly on the center crosshair.
Repeat from top ruler and drag out a horizontal guide. The center of your page is now defined.
Click the Move Tool (V) to cancel the crop. A dialog box will appear. Click "Don't Crop". There you have the center of the document.