Frontal collision: damage to the join of the top tube and head tube.
Take some string and check alignment from rear dropouts to head-tube. The string should pass equidistant from each side of the seat-tube.
Check that the seatpost clamp is not bent or damaged, and look for rust inside the seat tube.
You can generally tell the quality frame from the quality of the components like dropouts. If they look like investment-cast ones, not pressed steel, it should be OK.