I think it was a throwaway line by the instructor as one of the items on the checklist was "undercarriage" - obviously for those aircraft that have retracts, the 172 didn't, so he said "just look out the window and make sure the wheels are still there!"
(Obviously not suitable for most low wing aircraft!)
If I remember right, it was "BUMFHH" - Brakes, Undercarriage, Mixture, Fuel, Hatches and Harnesses.