You may or may not ever know whether your loan has been sold. If your primary lender is still servicing the loan, you will not know that it is actually being held by another company. If the buyer of your loan is servicing it or uses a different servicing agent, you will get a notice that your mortgage has been sold, and you should make future payments to the buyer or buyer’s servicing agent.
Regardless of whether or not your loan is sold, the terms of your loan will not change, and you will not have to pay more money to the new company just because your loan has changed hands. It is not something that you need to worry about. People often think that their payments are going to increase or their interest rate will rise if their loan is sold, but that is simply not true.