You probably had someone check your foundation when you bought the house, but have you looked twice at it since? These problems don't happen overnight.
They start small and they grow. By the time an issue comes to your attention, it might be too late. Learn to recognize the problems while they're still small and cheap. Your wallet will thank you later.
You might be able to tell if your floor is level just by walking on it. Some people have a knack for recognizing this problem. If you suspect your floor of being a bit off balance, grab a tennis ball and see if it stays put.
If a door has been scuffing the floor when it swings open, double check the floor. That could be your culprit. A slanted floor can be evidence of a serious structural issue. A shifting foundation can slowly pull apart your house's frame.
Sight Your Walls
Look down your wall from corner to corner and make sure there are not bumps or bulges. Walls should be flat. Grab a level and see if they're leaning at all as well. Problems with the walls can mean problems with the concrete.
Check Your Foundation
Walk around your foundation with a sturdy screwdriver, and poke your foundation firmly. If you can dig a hole in the concrete, that's a bad sign. It should be firm enough, and dense enough to keep you from causing any damage.
Just give it the screwdriver test though. If you start using the jackhammer test, you've gone too far.
Know Your Cracks
Concrete is always shrinking and expanding. It shrinks with the cold and then spreads back out in the heat. This can cause cracks, and most of these cracks are no big deal. Some of them, however, can be evidence of serious problems.
Small hairline cracks between concrete blocks are not a problem. Cracks on areas where the concrete stair-steps down to follow a slope are also not a big deal.
They are probably just caused by shrinkage. If you're worried about one, paint it over with some waterproofing paint, and make sure the crack doesn't come back.
Stair-step cracks on the walls are a bad sign, especially if the wall is bulging. Also look out for horizontal cracks, near the soil. This can mean the moisture in the soil is working into your foundation and putting pressure on the concrete. For problems like these, it's best to bring in a structural engineer.