Once you find out that a table is not used currently, the next question is "when was it last used?"
Get access to the database as a superuser or to the database host computer as user postgres.
PostgreSQL does not have any built-in last-used information about tables, so you have to use other means to figure it out.
If you have set up a cronjob to collect usage statistics, as described in the previous chapter, then it is relatively easy to find out the last change date using an SQL query.
Else, you have basically two possibilities, neither of which gives you absolutely reliable answers.
You can either look at actual timestamps of the files in which the data is stored, or you can use the