During the fox attack before the leaf village separated the uchiha clan from the rest of the village the Uchiha clan wasn't present. Now here’s the punchline, THE UCHIHA WERE KONOHA'S POLICE FORCE meaning their single role is to protect the village from harm.
Not only the Uchiha weren't present but suspiciously sasuke's parents as well. Sasuke's dad was the leader of konoha's police force, leader of Uchiha clan and incidentally the mastermind of the revolt that were told by Uchiha Madara came about in response to konoha's suspicions after the fox attack.And Kushina told Sasuke's mother that she was going to give birth. It was confidential information and no one but third/forth Hokage and wives, elders and third hokage's personal security crew knew. Then an Uchiha, Uchiha Madara frees the fox, the same day KUSHINA TOLD SASUKE'S MUM SHE WAS GIVING BIRTH and incidently along with her husband ARE NOT PRESNT THE NIGHT THE FOX IS RELEASED Hummm that's quite the coincidence wouldn't you say. Back home they CALL that prime suspect.
Itachi mentioned his parents wasn't in the village at the time, but look at what he said in more detail. "WHY DID MUM AND DAD HAVE TO GO OUT NOW" BY THE WAY IT WAS THE MIDDLE OF THE NIGHT when people usually sleep, particularly mothers with newly born babies who aren't on ninja duty. We didn't see Kurenai after she just gave birth fight Pain during the invastion did we?.
SO where were the Uchiha clan(POLICE FORCE OF KONOHA) during the village's DARKEST HOUR?
Did Madara do something to them?
Were they working with UCHIHA MADARA who told them to revolt during his time at the village? To only backstab them afterwards as he hated them just as much as konoha as he tried to get his clan to revolt during the leaf village's formation.
Note: The 4 things that aided minato to formulate his thoery that the masked man was madara are the following.
1) The masked man knew the seal was at its weakest during child birth.
(This indirectly could mean Madara knew Kushina was the fox's host already SOMEHOW and Kushina had no idea how Madara knew the seal is at its weakest during childbirth. But Madara does seem to know some things about the uzumaki clan like in Uzumaki Nagato's case.)
2)The masked man slipped through all the defences the third hokage set up, eg Konoha's BEST barrier as this was outside the village, secruity anbu etc..
3)The masked man has strong time-space jutsu
4)The masked man could control the fox
Kushina did say it was top secret that she was the fox's host only the third/forth hokage and leaf elders knew. But this doesn't explain how Madara knew WHERE and WHEN the child birth was taking place.
So how did Uchiha Madara find out?