If you use the lifts it is theft of services and you can be charged. Mountain ops will call the police in these situations to make an example of you and will press charges.
Skinning/hiking up to mid mountain lifts - patrol/mountain ops watches for this (with cameras at some resorts) and you can get caught. Whistler and Sunshine are two examples in Canada where they do watch for this. Backcountry one ride pass, probably little chance of getting caught there as long as you actually leave the boundary and come back later.
Even if the resort is in a national/prov park, the operator has certain rights to restrict movement/use of the ski resort, they may have to grant you right of way to beyond the boundary, but you can't just skin up and do whatever you want (e.g. skin up the middle of a busy groomer, ski in their terrain park, closed terrain etc).