If you think the performance of the query will be better using the index, how could you force the query to use the index?
First you would of course verify that the index gave a better result for returning the complete data set, right?
The index hint is the key here, but the more up to date way of specifying it is with the column naming method rather than the index naming method. In your case you would use:
select /*+ index(table_name (column_having_index)) */ *
from table_name
where column_having_index="some value";
In more complex cases you might ...
select /*+ index(t (t.column_having_index)) */ *
from my_owner.table_name t,
...
where t.column_having_index="some value";
With regard to composite indexes, I'm not sure that you need to specify all columns, but it seems like a good idea. See the docs here http://docs.oracle.com/cd/E11882_01/server.112/e26088/sql_elements006.htm#autoId18 on multiple index_specs and use of index_combine for multiple indexes, and here http://docs.oracle.com/cd/E11882_01/server.112/e26088/sql_elements006.htm#BABGFHCH for the specification of multiple columns in the index_spec.