One's Prayer Being Accepted

They said about Ben Dosa that he would pray on
behalf of the sick and immediately after his prayer.

Ben Dosa:	This one shall recover from his illness and live and this one shall die.
People:	From where do you know?
Ben Dosa:	If my prayer is fluent in my mouth as I recite it and there are no errors, I know that my prayer is accepted. And if not, I know that my prayer is rejected.