If you know the index name then:
SELECT pg_get_indexdef('your_index_name'::regclass)
will do.
In case you want a full list of indices for a certain table:
SELECT c2.relname, pg_get_indexdef(i.indexrelid, 0, true), c2.reltablespace
FROM pg_catalog.pg_class c, pg_catalog.pg_class c2, pg_catalog.pg_index i
WHERE c.oid = 'your_table_name'::regclass AND c.oid = i.indrelid AND
i.indexrelid = c2.oid
If you have more questions of that kind try starting psql with -E option
which enables internal queries' display (this is what I did).