The list is long. If u are buying for first time, do take denter & some one who can check that car reg book is ok.
Match chassis & engine no with that on the book. (also check that chassis & engine no are not messed with on vehicle), clear if from rescue 15
leave the body check of car to denter as everything have to be inspected from minor scratch to major, but not all denter are good enough to catch accident car.
aur ha for checking of engine healthiness do check blow by. As i say list is long, so best of luck as buying a used car is luck as well.