Perhaps one of the most telltale signs that your home may be settling are cracks between the crown molding and the ceiling. Appearance of some cracks is perfectly normal. However, if you see a noticeable amount of separation between the ceiling and the crown molding it can be cause for concern. Cracks in excess of 6 mm (1/4") should be considered a sign of abnormal settling.
Another sign of abnormal settling is curvature in your ceiling. This is especially noticeable in the corners of a room where the walls meet the ceiling, creating what should be a clean edge. If you notice rooms where the ceilings seem to bulge or seem saggy, this is another indication that your home is settling in a way that is not normal.
The walls in your home are always on display and can provide you with the most obvious indications that your home is settling in a way that is not acceptable. Walls that seem to softly buckle can be a clue that there is an abnormal pressure on the structure of the home. This pressure could come from the foundation settling or it may be something as simple as having an imbalance in the amount of furniture in the rooms of your home. Take note of which sides of your home hold the majority of your major appliances and furniture. If there is a sharp imbalance, distribute your furnishings more equally.
A settling home can lead to cracks in plaster and holes in the materials your home was built with due to the shifting in weight. This can spell big trouble during a big storm or a flash flood. Finding water in your basement or crawl space is a clue that your home isn't being protected from the elements. When your home is susceptible to moisture it can lead to additional problems such as mold.
So what should a concerned homeowner do when he finds evidence of settling? First, contact a home inspector for a second opinion. If, however, you want an expert opinion, consult with a structural engineer to determine whether the settling you see is normal or potentially dangerous.