The EXPLAIN plan shows all the partitions scanned for a query. If you run the EXPLAIN plan for SELECT * FROM <table>, it lists all the partitions, for example:
mysql> EXPLAIN SELECT * FROM employees\G*************************** 1. row *************************** id: 1 select_type: SIMPLE table: employees partitions: p1980,p1990,p2000,p2010,p2020,pmax type: ALLpossible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 292695 filtered: 100.00 Extra: NULL1 row in set, 1 warning (0.00 sec)