Have you ever wanted a ruler on your screen to make sure two things were aligned properly? If you're running OS X, you already have a tool to do this: the screenshot tool.

The key combination you want to use is:
⇧+⌘+⌃+4 (that's Shift+Cmd+Ctrl+4)

After you've done that your mouse should be a crosshair. Click and drag between the two areas in which you're checking alignment. If you release the mouse button, a screenshot of whatever is in that box will be saved to the clipboard.