DBA_DEPENDENCIES won't give you tablespace_name, so it's hard to figure out if any objects in that particular tablespace has dependencies. Since your error message is pointing to some index, I'd either check dba_indexes or dba_tables to look for dependencies in that tablepsace.
Originally posted by slimdave
You basically have dependencies between objects in the tablespace and objects outside of the tablespace.
For example, you might have a partitioned table with some partitions in that TS and some in others.
You can use DBA_DEPENDENCIES view to help find these objects.