Listing active PostgreSQL foreign data wrappers
The information you're looking for is in various pg_catalog tables. pg_class is the main table that represents all relations, normal or foreign; you can limit it to foreign tables by including WHERE relkind='f'. Information on the foreign data wrappers themselves is in pg_foreign_data_wrappers, on servers in pg_foreign_servers, and on foreign tables in pg_foreign_tables.
Start with select relname from pg_class where relkind='f' to just get a list of table names, and from there you can use the pg_catalog docs to find whatever other pieces of information you need for what you're trying to do.