Usually the term "Shifted Belt" means irregular wear or a belt separation.
First, the term "Shifted Belt" seems to derive from a manufacturing technique where the belts were applied to bias belted tires and during the expansion process, the expansion would not be even, resulting in a belt that "shifted" off center (but evenly around the tire), or more in one spot than the other. The off center version resulting on a pull and the uneven version resulted in a funny wear pattern in one or more spots around the tire. Because of this, many folks still use the term anytime there is irregular wear, in spite of that fact that the manufacturing technique requires little or no expansion and the belts don't "shift" during manufacturing.
I should also say that belts can't shift after they are manufactured. They are encased in rubber, which holds them in position. Some folks may say that "flat spots" indicates that "belt shifting" is possible, however, flat spotting is not the rubber (or belts) moving, it's the rubber taking a "set" when under compression. We engineers call this "Compression Set".
I think the question you are asking is either:
1) How can I tell if I have a separation? A belt separtion will cause a vibration that is small at first and gradually gets worse. In the later stages, a bulge may be found. Lack of a bulge doesn't mean the tire isn't separated. You might also see excessive wear on one of the shoulders in a location about 8 inches long. This last symptom is what is probably the source of confusion about "shifted belts"
2) If you have irregular wear - one sided wear or wear where a tread element is morw worn than the element next to it in a ore or less cyclical pattern around the tire - then you are looking at an alignment issue.
One side wear is usually camber related, although I have seen some combinations where toe was the main contributor.
Irregular wear is usually toe related.
Low inflation pressures and lack of rotation aggravate the situation.
Unfortunately, vibrations that get gradually worse can mean either a separation is developing or the tire is developing irregular wear. It's best if you have a question to have the tire looked at. As a word of caution - there are a lot of people working at tire stores that can't correctly diagnose the situation, so take what they say with a gain of salt (and some, you need the whole shaker!).
Hope this helps.