What rate of faculty turnover is healthy?
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.