As a flight instructor there are certain things you learn to ask over time. Commonly I see students either miss or skip key items during the preflight.
1. Aircraft Documents – Sometimes when I’m sneaky I’ll pull out the aircraft’s registration or weight and balance. My new students find it almost every time. However my more seasoned students (maybe out of complacency) almost never catch this.
2. Pitot Tube – I seen crazy things in pitot tubes before bugs, dirt, water. It’s important that you not only check the ram air hole but also the drain hole to make sure they’re both free from any blockage.
3. Counterweights – Although not on the checklists, it does say “control surfaces” the counter weights are part of that. A typical Cessna 172 has 3 counter weights on each aileron. You can find them by lifting up the aileron and looking towards the wing tip. Flying with unbalanced controls could spell disaster.
4. Brake lines – You may check the tires for flat spots and look at the brakes to make sure there’s plenty of brake pad. Have you ever considered a brake line leak? Brake fluid is pink in color and would be found in small puddles directly beneath the brake or wheel strut.
5. The bottom engine cowl – I always encourage my students to learn all they can about the powerplant, after all it’s the heart and soul of an aircraft. If a cylinder has a worn gasket, or a line has a leak where will it go? Either the hanger floor or the bottom of the engine cowl. Don’t be afraid to stick your head inside the engine. Ask either your instructor or an A&P to go over everything with you. 
You’ll be amazed about how much you’ll learn.
These are just a few things I either see students miss or never instructed on how to do properly.
What are some of your pet-peeves for preflight’s?
