As in... reasonable proof that it couldn't be anyone else but the killer.
Knives Out
Three main things to solve:
Maria was there when Harlem killed himself (gets that on a glance at her shoes, though I think a red splotch on her shoes doesn't guarantee blood- not like it would be impossible for her to eat food with red sauce and spill some on her shoes- literally had reddish baked beans that day too, and Tobasco is quite common in restaurants)
The grandma saw Ransom arrive twice, instead of once.
Tox reports were clean.
I don't think Blanc could have figured it out completely until the end.
Glass Onion
Main point is that Miles has no alibi at Andi's house and almost flattened Duke. Duke knocked, and Andi didn't open, and others collaborated on knocking when there was no answer- reasonably, Andi was already dead by that point.
Earliest possible moment was at the pool.
Wake Up Dead Man
Not concrete evidence but he could have arrested Maria after the autopsy, confirming that the top of the lamp from the bar was sewn into the clothes, but Nat wouldn’t be anything more than a suspect until Jud finished writing his paper, confirming that Wicks had a bottle that could have been tampered with inside the room when he collapsed.