Great idea to buy an unlifted Jeep. I bought mine in pretty much new condition with 18K miles. Some Florida resident gave it up without ever taking it off-road. Can't blame him/her, not a great choice if you do not use it as intended.
As far as identifying a lifted Jeep... I would just look for signs of (ab)use, basically at the chassis. Scratched differentials, skid-plates, bracketry, or even bent items are a dead give away. If a Jeep was lifted and not off-roaded I do not see a reason to not buy it. I know that I for one would not be interested in my Jeep in the condition it is now.
You can always replace a lift if you want to put one in.