What is considered a reasonable rate for faculty turnover in a "healthy" department? How much turnover is too much/little? Presumably the rates might depend on seniority. Does departmental turnover tell you anything about your colleagues?
Reasons for faculty turnover are more important than numbers. For example, in the late 90s, CS professors were leaving their positions to start companies. I also know of professors who left otherwise ideal positions to solve a two-body problem (move near their spouse).
One important component of faculty turnover is tenure denials. If you are a serious candidate, you should ask why your opening exists and the outcome of recent tenure cases.
Another component is faculty taking advantage of early retirement offers. If a school is offering sweet deals for retirement, it may not be at its most financially secure. (On the other hand, if they're doing a tenure-track search, that's a positive sign.)
If you can contact faculty who have left through your social network (e.g., if one of your committee members knew them socially), you might be able to get the dirt on the department.
It also doesn't hurt to look at the student newspaper, now usually available online. At one school at which I interviewed, my host stopped me from picking up a copy. I did so later and read about major problems between the faculty and administration that involved a lawsuit, the AAUP, etc.
When I was actively in the job market, I noticed a small number of schools that seemed to have multiple openings every year. It made me wonder what was going on at these places, but I had no way of knowing. At my current school, we have several retirements coming over the next two to three years. Multiple hires for several years in a row. Now, I know the reason behind the situation, but will prospective hires? Is this a healthy situation? There are various scenarios that could be playing out. Espertus has given you several good ones. My example is another. Try your best to find out what the cause is, but it is likely to be difficult to get a definitive answer. The best advice is to keep your eyes and ears open (which you seem to be doing) and try to read between the lines in conversations. Use what sources you can to get information that will help you.
Not a complete answer, but a possibly instructive example (at least ground for discussion). The math department at Ecole Normale Supérieure has a somewhat radical approach to this question. Two rules for the teachers and researchers there:
No one stays longer than ten years
No one teaches the same class more than three years in a row.
The idea of course is to keep the department members "fresh", on the cutting edge of research and to vary the topics that are represented. Such a strict rule is clearly not practical in any but this very specific environment, but I think the idea that there should be a minimal turnover is a sensible one.