The volume buttons are slightly offset and the breaks in the aluminum band around the phone are slightly different. That's pretty much it. I'd have to look at them side by side, but there may be a slight visible difference in the camera lense as well.
Only very slight differences. If it is a CDMA model, you can tell because the iPhone 4 does not have a sim card tray while the 4S does. On the GSM model, I believe the only difference is a slight difference in the appearance of the antenna.