getting innodb buffer usage
SHOW GLOBAL STATUS LIKE 'Innodb_buffer_pool_pages_free' and
SHOW GLOBAL STATUS LIKE 'Innodb_buffer_pool_pages_total' will enable you to derive this percentage.

Keep in mind a page_free of 0 isn't fatal or even necessary bad - it just means every once in a while a read will come from disk or page cache. Its the rate at this occurs that is more important to performance of related to the size of the innodb_buffer_pool.