There doesn’t seem to be a way to determine who’s Imperiused.
There doesn’t seem to be any magical way of determining if someone was or is under the Imperius Curse. After the first war against the Dark Lord, the Ministry has difficulty figuring out who had acted of their own free will and who had been under the Imperius Curse. If there was a magical way of determining who was Imperiused, figuring it out wouldn’t have been so difficult for the Ministry.
“Years back, there were a lot of witches and wizards being controlled by the Imperius Curse,’ said Moody, and Harry knew he was talking about the days in which Voldemort had been all-powerful. ‘Some job for the Ministry, trying to sort out who was being forced to act, and who was acting of their own free will.”
- Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)
In addition, the Ministry wasn’t always right in their final decision. Some of the Death Eaters were able to claim they were Imperiused and get themselves cleared despite having acted of free will.
“Avery – from what I’ve heard he wormed his way out of trouble by saying he’d been acting under the Imperius Curse – he’s still at large.”
- Harry Potter and the Goblet of Fire, Chapter 27 (Padfoot Returns)
Both Avery and Lucius Malfoy were known to have gotten themselves acquitted this way. If there was a magical way to determine if someone was Imperiused, it should have a better success rate.
It doesn’t seem likely this would change if the person is dead.
There’s nothing in canon that implies this would change and the Imperius Curse would become detectable by magical means if the person who was possibly Imperiused was dead. The Imperius Curse doesn’t seem to leave a detectable presence on living people, since it’s clearly shown that there isn’t a reliable way of determining whether someone was under it, and there’s no reason given to think it would suddenly become detectable after death. It seems much more likely that the Imperius Curse would still leave no trace after the person’s death.