After a lot of debugging and numerous searches on the Community, I wanted to post what seems to be the only working solution I've been able to come up with for using User Criteria to limit something via a Condition, such as a Header Menu via the Service Portal.

SNC.UserCriteriaLoader.getAllUserCriteria().toString().includes("sys_id_of_the_User_Criteria_record")

Note that you don't need the User Criteria for the Service Portal plugin for this to work.