I don't know how the adcoms get this information, but they definitely have it prior to May 15 - i.e. now. I don't know if they know what other schools have offered you an acceptance, but they most definitely know where you are holding spots in other classes. I specifically asked the dean of admissions at one of my interviews about this. He told me that in spite of this knowledge, holding other acceptances was not a factor that deterred this particular school from offering you an acceptance.