Or how did Sozin know the Avatar wasn't among the airbenders that were killed-off during the genocide? In his will, Sozin made clear that the Avatar, who's the last airbender, escaped from him, respectively the Fire Nation army. How did Sozin know that and how did he want to make out the Avatar among the airbenders?
The Avatar's identity is revealed at age 16, in Aang's case at age 12 but the Fire Lord didn't know that. Did Sozin prepare a trap to let him enter the Avatar state, thus recognizing him, for instance?