Leaks, moisture and water damage. Paint and wallpaper can cover all manner of sins, and wet spots on a ceiling can be covered up without the underlying roof or skylight leaks ever being addressed. The only way to discover those is through the eye of an experienced handyman or roofer, which we suggest trying to do (remember to check for potential ice dam spots, since some roof issues don't present themselves until winter). If you have a client who is trying to cover up leaks, urge them to fix it. Most minor leaks are inexpensive to fix permanently.
More problematic -- and more expensive to fix -- is the issue of moisture, which can lead to one of the worst problems a home can have: mold. Rather than looking for signs of leaks, however, with moisture, you need to follow your nose. The telltale musty smell of a damp crawl space or regularly flooding basement should be obvious to most people. Pro tip: Don't take scented candles at face value. They could be covering something up.
Again, paint and sealant can temporarily cover up a lot of problems, and the structural integrity of a deck or the inner structure of a house is not always apparent on a first glance. Incorporating a termite inspection into any pre-purchase agreement is wise, and getting a deck expert to take a look at major outdoor structures could pay off in the long run.
A neighborhood may LOOK nice and have a great reputation, but every block has its issues and they're usually not happening from 2 to 4 p.m. on a Sunday during an open house. Do the next door neighbors have 10 teenagers cars parking up and down the block every night? Do they throw loud pool parties every weekend? Does the dog behind you bark at all hours of the night? Does the guy across the street have an affinity for AC/DC and garage speakers? Making a few random "spot checks" of the property on weeknights and weekend evenings, or even walking the neighborhood and visiting with other residents, will give you a truer picture of the property.
A bit more concerning is the possibility of hazardous materials in the home (asbestos, lead paint) or in the yard (oil, chemical dumps). Older homes used to run on oil, which included tanks buried in the yard. Evidence of some of those tanks may have been covered up over the years and harmful chemicals could still be in the ground, which is bad news for kids, pets, and gardens. Look for a fill pipe sticking out of the ground -- and look closely. It might have been sawed off.
A quality inspection will turn up serious foundation flaws, but they're not as hard to spot as you might think. Small jagged cracks in walls -- especially from the ceiling down to the corners of door frames and windows -- are telltale signs, according to Realtor.com (link below). Look closely; a quality paint job can cover those up.
This goes beyond aging furnaces and air conditioning units. Finding an average utility bill should be standard research for you, and any inspector worth his or her salt will be able to tell you that a unit is on the fritz. Harder to get an idea for, however, is the quality of insulation, ductwork and general airflow around the house. We all know it to be true: A hot bedroom, even with a ceiling fan, can be unbearable, while an always-freezing dining room or living area can scatter the family to other corners of the house. When you're walking through, be sure to do it at different times of day and ask to have the temperature adjusted, or know what temperature it is set to. Most of all, to take off your coats and pay close attention to how they feel in certain rooms... to trust their instincts and think about their family's comfort.