sys.inode_number_from_rowid

Given a row, return its inode number (its shard location). See Querying Row IDs.
sys.inode_number_from_rowid(rowid)

This function accepts either the string rowid as its input or a specific rowid value. In most cases you will use the string rowid to return the specific row ID for each row in a set of rows. (If you specify an individual row ID as the input to the function, make sure it is valid; the system does not detect row IDs that are not valid or do not exist.)

For example, you can find out which shards are used to store rows for a given table:
premdb=# select distinct sys.inode_number_from_rowid(rowid) from newmatchstats order by 1;
 inode_number_from_rowid 
-------------------------
                    4097
                    8193
                   19457
                   19458
                   46082
                   68609
                  130049
                  146433
                  149505
                  196609
                  250881
(11 rows)