When an emphatic form changes from active to passive, its grammatical person and number stay the same.
PassiveActivePerson
لَيُفْعَلَنَّلَيَفْعَلَنَّmasculine Ghaa'ib singular
لَتُفْعَلِنَّلَتَفْعَلِنَّfeminine Haadhir singular
لَنُفْعَلَنَّلَنَفْعَلَنَّMutakallim plural
First identify the Seegah. Then use the verb's vowel pattern to decide whether it is active or passive.