My sorority requires a legacy introduction form to be filled out by the person introducing the granddaughter/daughter/sister as a legacy. (If that person is deceased, I guess it is filled out by someone else) it is preferably sent electronically to headquarters or can be sent "snail mail" to the chapter who then has it vetted at National that this indeed a legacy and then the chapter receives their list of legacies. I may have missed a few steps but this is generally how it works.
My sorority wants PNMs to fill out a legacy form too. It has to be snail mailed like our other recs do. I guess ours are checked too! I'm told that a fair amount of girls do try to falsely pass themselves off as legacies.