Filter > Blur > Average
Image > Mode > Grayscale
Now use the Eyedropper tool to sample any point in the image.
Finally, look in the Color Palette and see what percentage of gray is "average" for your picture.
If the answer is about 50%, you probably used auto-exposure. Phil likes aperture-priority automation. He justifies its use this way:
"I don't think the automation in the camera makes a great deal of difference. Once you have decided what you want to take a picture of, compose the shot and focus on the topic of interest there are only two real choices you can make on a camera; aperture and shutter speed. And the choice of one strongly constrains the other"
This is true - if you want the average density of your photo to be 50% gray. Your camera wants the average density of your photo to be 50% gray, because that's the average for a photo with a "full range of tones" more or less evenly distributed (for example, a picture of a human subject outdoors in the daytime on a grass lawn.)
Here's a photo I took at Reed's late last year. I saw some shadows on the wall in a dark corner and thought they looked interesting in a brooding, film-noirish way. The corner was very dark, and I wanted the picture to look dark, the way my eye saw it. If you run the Photoshop action I've described above on this picture, you'll see that its average gray percentage is 95%.
When I took the picture, I knew that the normal exposure for ambient light in Reed's is about 1/60 of a second at f/2 on 400 speed film. In dark corners, there's much less light. I shot this picture (manually) at 1/30 at f/2. If I'd used aperture-priority automation at f/2 (assuming my camera did that, which it doesn't), the camera would have noticed that the wall wasn't lit, and it would have set a shutter speed of either 1/4 or 1/2 second - resulting in a picture with an average gray percentage of about 50%, which would have looked like this:
I like my picture a lot better than the one an automatic exposure meter would have generated. I could have tricked my F-100's auto-exposure system into producing the picture I wanted by setting "exposure compensation" to tell the F-100 that the scene was supposed to be dark. But then what good is the automation? I already know the scene is dark, and it's no harder to set the exposure values manually than it is to set exposure compensation manually - so all the automation does is make it more likely that I'll get lazy and end up with a bad picture.
Automation won't hurt your "average" pictures (photographs of people in daylight, for example), because those are the pictures it was designed to produce. It is much more likely to hurt dark or light pictures, or anything else "out of the ordinary".
Open up a bunch of your pictures and try the Photoshop experiment I've described above. If your highest gray density is only 10% higher than your lowest, turning off autoexposure will probably improve your photography.