How do I determine the probable profitability of a proposed independant film?

Well, first you must determine your probable costs.  That's the easy
part.  What will it cost for film, processing, actors, technicians
etc.?
Then you have to determine your market.  Will the film be shopped to
various festivals?  Is it a niche film of the horror, action, gay,
etc. variety?  What are the chances that a chain will pick it up for
promotion after it is accepted for viewing at a festival (if that
happens)?
There are a LOT of factors to consider.