Make sure that you are a superuser, or at least have full rights to the table in question.

To see whether a table is currently in active use (that is, whether anyone is using it while you are watching it), run the following query on the database you plan to inspect:

CREATE TEMPORARY TABLE tmp_stat_user_tables AS
       SELECT * FROM pg_stat_user_tables;

Then wait a little, and see what has changed:

SELECT * FROM pg_stat_user_tables n
  JOIN tmp_stat_user_tables t
    ON n.relid=t.relid
   AND (n.seq_scan,n.idx_scan,n.n_tup_ins,n.n_tup_upd,n.n_tup_del)
    <> (t.seq_scan,t.idx_scan,t.n_tup_ins,t.n_tup_upd,t.n_tup_del);

The pg_stat_user_tables view shows the current statistics for table usage.

To see whether a table is used, you check for changes in its usage...