They knew because the FDS told them so.
Romans Chapter 8
12So, then, brothers, we are under obligation, not to the flesh to live in accord with the flesh;13for if ?YOU? live in accord with the flesh ?YOU? are sure to die; but if ?YOU? put the practices of the body to death by the spirit, ?YOU? will live.14For all who are led by God's spirit, these are God's sons.15For ?YOU? did not receive a spirit of slavery causing fear again, but ?YOU? received a spirit of adoption as sons, by which spirit we cry out: ?"Abba,? Father!"16The spirit itself bears witness with our spirit that we are God's children.17If, then, we are children, we are also heirs: heirs indeed of God, but joint heirs with Christ, provided we suffer together that we may also be glorified together.out
Without a doubt a person will know when they are chosen. The spirit compells you to do things in accordance with GOD's will.
Estephan
The G.B. claim to be chosen in order to claim a degree of authority over the congregation. They are not the faithful and discreet slave otherwise they would teach that the slave is only one person. There is no basis for their teaching that the slave is comprised of 144,000.