I hesitated before writing this question, as it sounds like I'm after an opinion without trying anything yet. That's not the case, I just don't want to go down the wrong route with architecture - it needs planning well in advance and I don't want to fall victim to the XY problem.
I'm pretty new to MVC, I understand WebForms carried a lot of 'behind the scenes' stuff in the view state, some of which may have been used for authentication? But I don't know how I can pass authentication data to an MVC controller. More importantly, I don't know if I'm approaching these requirements right. Should I have a separate 'offline' form that will never even attempt to send off the data to the server and send it later on from a different section? Or is it common to check if a user is authenticated or not from their AJAX request?