How to determine when an index should be rebuild?

 Nightly a cube build occurs on the RAC database.  The cube is a manul rollup that rebuild tables an summaries.

As part of the nightly rebuild, the cube table is dropped along with it's indexes.  Then the table is repopulated and

the indexes are created.  The recreation of indexes accounts for a third of a 3 1/2 hour build of the cube.

Is there a script that would detect when indexes require rebuilding?  That is only find the necessary indexes that

would benefit from a rebuild.   The cube building process urks me, I have reduced the procedure from 6 hours to

3 1/2 hours by tweaking the scripts.  I would like to further reduce the build down to about 2 hours if I could.