You can verify that indexes are being used for similar searches of vector data by checking the access plan in the EXPLAIN statement.

Example) Access Plan for Vector Similar Search

EXPLAIN SELECT * FROM items ORDER BY embedding <=> '[3,1,2]' LIMIT 5;
                                      QUERY PLAN
---------------------------------------------------------------------------------------
Limit  (cost=16.60..16.81 rows=5 width=33)
   ->  Index Scan using idx_hnsw on items  (cost=16.60..437.60 rows=10000 width=33)
         Order By: (embedding <=> '[3,1,2]'::vector)
(3 rows)