you can find lots of info if you search these forums but generally look for oil leaks, coolant colour, engine noise, interior condition such as seat and carpet wear, take it for drive and check wheel alignment, driving noises etc.
check to see if any panel work has been done by comparing the panel paint texture and variations in shine and colour, see if the doors shut and open nicely, check for even gaps between panels and so on.....check the under car for rust and grease and evidence of repairs...
but most importantly have it checked by a professional....and good luck!!