At one place there is a hadith which says people who die upon fatrah will be tested on day of judgement.
The man who died during the fatrah will say, ‘O Lord, no Messenger from You came to me.’ He will accept their promises of obedience, then word will be sent to them to enter the Fire. By the One in Whose hand is the soul of Muhammad, if they enter it, it will be cool and safe for them.”
According to another report, he said: “Whoever enters it, it will be cool and safe for him, and whoever does not enter it will be dragged to it.” (The hadeeth was reported by Imaam Ahmad and Ibn Hibbaan, and deemed saheeh by al-Albaani, Saheeh al-Jaami’, 881).
At another place it is mentioned that last prophet's parents who died between fatrah will go to hell without testing.
Muslim (203) narrated from Anas (may Allaah be pleased with him) that a man said: “O Messenger of Allaah, where is my father?” He said: “In Hell.” When he turned away he called him back and said: “My father and your father are in Hell.”
Muslim (976) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “I asked my Lord for permission to pray for forgiveness for my mother, but He did not give me permission. And I asked Him for permission to visit her grave, and He gave me permission.”