What determines the satellite beam you're using?

I know that satellites have several spot beams and that each of them are normally focused on a different region of the earth. However, in my case the particular satellite that I'm using has two spot beams that overlap each other a little bit and I'm located within the overlap. How would I know which spot beam I'm using? Would it even matter? Would it be determined by the satellite transponder that my transmit frequency is on? Could my signal be jumping back and forth between the two spot beams and therefore be effecting my service?