I dont see any long running queries under pg_stat_activity or pg_stat_statements.
With pg_stat_activity, you would need to catch them "in the act", but there are no columns there which describe temp file usage anyway.
pg_stat_statements has the columns "temp_blks_read" and "temp_blks_written", in all supported versions, so you should be able to spot the queries using temp files there. Unless maybe your pg_stat_statements.max setting is too low and those queries are forced out.
Or you could set log_temp_files = 0. That would log all statements using temp files into the PostgreSQL server log.
