Two ways that I know of. #1- phone any dealer with your VIN and ask the parts dept. to look it up. #2 - Gently pull the mirror glass from the inner area closest to the door glass and look for two small wires attached to the mirror glass. be very carefull not to pop the glass off it's attachment. you shold also have a flash light to light the area. If there are no wires it's not heated.
The heated mirrors are part of the "Mirror group option" this is real easy to tell, with this group option you get-- heated mirrors, auto dimming rear mirror with dual reading lamps. Now I have seen you can get the dual reading lamps but still not the auto dimming feature and without the auto dimming feature you dont get the heated mirrors(I saw this when l was looking for my 2006SS I came across this and it did not have the heated mirror or dimming feature) So if you dont have the auto dimming rear mirror you dont have heated side mirrors its that simple. So if you have that little flick switch under your rear mirror to get the high beams that dork is using behind you then your out of luck. I know that this is valid for all impalas from 2000 to current-2010.