This one helps you when you are in doubt if some obscure table is used any more, or is it just leftover from old times that just takes up space.
Make sure that you are a superuser, or at least have full rights on the table in question.
To see if a table is currently in active use, that is, if anyone is using it while you watch, run the following:
create temp table tmp_stat_user_tables as select * from pg_stat_user_tables;
Then wait a little, and see what is 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);