Connect with the postgres server and run the \dn+ command to check schema permission

 sudo -u postgres psql

 \dn+

In PostgreSQL 15+, pg_database_owner controls the public schema. The =U indicates that PUBLIC has USAGE permission. In earlier versions, =UC meant PUBLIC also had CREATE permission.

For review PUBLIC permission on tables following query is used:

SELECT grantee, table_schema, table_name, privilege_type 
FROM information_schema.role_table_grants 
WHERE grantee = 'PUBLIC' 
AND table_schema = 'public'
LIMIT 10;

Revoke PUBLIC schema permissions for better security

Revoking USAGE from PUBLIC allows only specific users to access the schema.

REVOKE USAGE ON SCHEMA public FROM PUBLIC;
GRANT USAGE ON SCHEMA public TO app_user;
GRANT USAGE ON SCHEMA public TO readonly_user;

Verify again

Note:  PUBLIC now has no permissions on the schema