The protocol settings are stored in a multivalued AD attribute called (obviusly enough) protocolSettings, so use ADU&C to do a Query, select Custom Search in the Find box, and enter the following LDAP search criteria:
(objectcategory=user)(protocolSettings=*HTTP§1§1§*)
and all your OWA-enabled users will be listed for you (you can Add the Display name into the columns viewed to make sense of the list!).
To invert the search (ie find all users who have OWA disabled), change the first 1 to a 0, ie HTTP§0§1§
Incidentally you can use the same command for the other protocols, substitute the HTTP with POP3 or IMAP4.