I've owned my house for 18 months and the builder just sent me an enerystar certificate. It states I've got Low E rated windows...shouldn't I see some sort of marking on the actual window to indicate this?
Not really if the NFRC stickers have been removed. You might be able to find a serial number and contact the manufacturer about them and go from there.
You other option is to hold a lighter up to the glass and look for a reflection of the flame insided the window. That will indicate the presence of Low-e