You could put a hidden value in each form and determine which was posted by checking the value. But I agree with @Zuluru, it will be better to have two endpoints.
You could put names on the buttons too, and check that, but these are two fundamentally different operations, and thus should be handled by completely separate functions.
In your case you are working with Subscribes data and Contacts data. Typically, these two tasks would be performed in two different Controllers.
Write a SubscribersController action to handle one forms data and a ContactsController action for the other. Make sure the forms point to the proper new endpoints and you’re good to go.
In the case that you’re creating the same type of record for both, and just taking more data from contacts than subscribers, then the functions could reasonably both be in the same controller, but definitely different functions.