im not sure if this is a real test... and Im really tired right now but if you go to somedomain.com/~someuser
if webusers are disabled it gives a 404 not found error instead of a 403 forbidden. (this assumes that directory listing is disabled and there is no index.htm in the webusers root) I guess you could just setup a webuser in plesk under a domain and then put an index page in the root and then see if it displays.
Sorry not much help. im a windows guy lost in a sea of plesk/linux