How to tell which indexes are not being used
You can monitor the index usage through pg_stat_user_indexes and pg_statio_user_indexes

More details about the statistics collector can be found in the manual: http://www.postgresql.org/docs/current/static/monitoring-stats.html

You should be careful with dropping unused unique indexes though. They might not be used for reading, but they are most probably vital to the integrity of your data.