In DB2, what are the alternative ways of evaluating an index's (or several indexes) benefit?
The best way to do this for a DB2 database running on Linux, UNIX or Windows is use the DB2 Design Advisor.  

This tool (included with DB2) allows you to specify a workload (which could include hundreds of queries), and it will analyze the workload and the existing database to recommend indexes and/or other physical design changes to optimize the workload.