PostgreSQL: How to view current "grant usage on schema" permissions?

Is there a system view in Postgres where I can see which users/roles have "grant usage" and on which schema?

>Solution :

The grants are stored in ACLs (=Access Control Lists) for each schema. You can view them by looking a pg_namespace

select nspname, nspacl 
from pg_namespace

The format of the acl is explained in the manual