Foundations are just that -- foundations -- any building is only as strong as the foundation that it sits on. In this article I will focus primarily on residential houses, as they are the most common; however, the simple methods may be applied to any freestanding structure. There are several tell-tale signs of foundation trouble that are easy to spot, here's how...
The first thing to check are the interior windows and the general area around them. Look for cracks in the drywall near the top two corners of the window. It they are there they should be easy to spot and usually run up and slightly sideways of the window until reaching the ceiling. If the drywall/sheetrock is obscured by paneling, or something similar, then peel back or remove the paneling to get a clear view of any cracks that may be there. Also check the bottom two corners for similar cracks that run down to the floor. Cracks indicate some sort of structural stress or damage. In order to determine if it's a minor or a major problem, it's a good idea to check all windows in this way. If more have cracks than do not, it's a decent bet that significant foundation problems exist. If only one or two windows have cracks, then it's possible that the damage is minor and localized. Many old houses and buildings will naturally settle on their foundations and cause a few minor cracks.
Much like before, the next thing to check are all the interior doors: bedroom, bathroom, closets. Check each one to see how they open and close, see if they all match up evenly with the door frames. If they don't shut all the way, or need to be forced to shut completely, then it's a possible sign of foundation trouble. Just like in step #1, it's all about how many doors do or do not shut properly.
The third thing to check out is the floors in each room. Take a standard carpenter's level and check all four corners and the center of each room in the structure. If the floors are more than slightly off level then foundation damage is possible.
The fourth and final check is the most telling of all. Carefully look over the roof line and sides. Notice if their are any sunken spots on the sides of the roof. Look closely along the top of the roof line to see if there are any bowed or crimped lines along the roof line. If there are any sunken spots, then this is an almost certain sign of foundation damage. If not, then the existing foundation is probably pretty solid.
Hopefully these easy steps will be helpful in accurately assesing the condition of your building's foundation!