Aperture 3 has a nifty feature that allows you to enable an overlay that shows focus points used by the camera when recording the picture. It also highlights which point was active. Granted, this feature could be viewed as a novelty, but I do find myself checking it.
In this shot, for example, I see all the focus points for the Canon 5D Mark II with the center point outlined in red. That was the point that I used for focusing. What you don't see, however, is where I directed that point when I composed the shot. The overlay pattern is always in the same place, as it appears when I look through the camera's viewfinder. Since I often use focus lock, then recompose, the overlay doesn't show me the actual object that I focused on.
In order for this to work at all, your camera needs to save the autofocus metadata to EXIF, and of course, it has to be compatible with Aperture 3. I've also learned through testing, that if you use the referenced file approach for library management, your masters have to be available to activate the focus points overlay.
To enable this feature, just go to View > Show Focus Points, or click on the icon in the metadata inspector (as illustrated in the photo). The keyboard shortcut is Option-F.
I think Show Focus Points is most handy when you're trying to analyze what went wrong in a misfocused shot. If your subject is on the right, and the red focus point is on the left... well, that might be a clue.
More Aperture Tips and Techniques
To learn more about Aperture 3, check out my Aperture 3 Essential Training on Lynda.com. Also, take a look at our Aperture 3 Focus Section. Tons of free content about how to get the most out of Aperture.