About two years ago I figured out that my iMac was not going to sleep even though the screen was blank and the LED was on. A bit of sleuthing brought out that a constantly-on LED meant only the screen was sleeping, while a dimming and brightening LED meant the system was sleeping. My LED was constantly on! It took some more detective work to figure out that my mail program, Eudora, was the problem. Qualcomm had stopped supporting it, so it hadn’t been updated to work with the new Mac OS (10.3?) and prevented the Mac from sleeping.
It’s therefore particularly annoying that my new iMac dispensed with that LED entirely, so now I have no way of knowing its sleep status! I currently have my power meter on a new external disk drive and can tell when the Mac is sleeping because then the drive also goes to sleep and draws less power. And there have been times lately when it should have been sleeping based on no activity within the specified timeframe, but was still awake at the system level. (The screen was blank, though.)
http://www.deviceguru.com/energy-star-or-black-hole/