In addition to the operation statistic counts, there are also a set of counters in the database that concentrate on physical I/O in pg_statio_user_tables (with all/system variations, too). The first pair of fields monitor use of the shared_buffers structure that caches table data, which is called the heap in this context. When a read happens, the database distinguishes between whether that read could be satisfied using a block already in the database buffer cache (a heap block hit), or whether it required an OS read to satisfy:
pgbench=# SELECT relname,cast(heap_blks_hit as numeric) / (heap_blks_hit + heap_blks_read) AS hit_pct,heap_blks_hit,heap_blks_read FROM pg_statio_user_tables WHERE (heap_blks_hit + heap_blks_read)>0 ORDER ...
