لَا makes both active and passive forms negative, so the voice is identified from the verb itself.
MeaningTypeExample
he does not helpactive negativeلَا يَنْصُرُ
he is not helpedpassive negativeلَا يُنْصَرُ
you (one woman) do not helpactive negativeلَا تَنْصُرِينَ
you (one woman) are not helpedpassive negativeلَا تُنْصَرِينَ
Key clue: لَا does not tell you the voice; the Mudhaari' vowel pattern does.