Most places with park passes have someone at the top of the park or at the lift servicing the park checking for passes. It works to a certain extent. It depends on the system the mountain uses to obtain them. At camelback, you need to watch a video and stuff, and that alone turns most people away who should not be in there. Creek makes you take an online test thats so easy a first grader who never rode before could pass, thus really not doing anything to curb the problem.
Most resorts who implement passes have a beginner park located somewhere else on the mountain that does not require a pass so all the noobs have something to learn on, without getting in the way/creating a dangerous situation for those who are at a higher level.