what to test for in private methods on c#?

I am new to c#. I do have some experience testing javascript. I was looking at assessing this test similar to checking for a method to be called in with a spy via sinon.

However, I am not truely sure what to test. As I noted I am pretty new to c# and am doing testing to get a feel for how the code works.

Here is the private method that I need to test for

 private TicketResponse.PaymentType GetPaymentType(TicketResponse.FormOfPaymentCreditCard paymentType, VCR_DisplayRSTicketingInfosTicketingInfoTicketingTicketDataTourInfo tourInfo)
    {

I read that I test the interfaces. So I am testing something along these lines?

 public enum PaymentType
{
    CASH, CREDIT_CARD, NITP, TRAVEL_CREDIT, GIFT_CARD, JETBLUE_REWARDS, NONE, CHECK, EXCHANGE
}

Any insight would be super helpful thanks.