I wonder why the Veritaserum (The Truth Potion) was not used on Harry or the others to check whether Voldemort's back or not to prove for those who didn't believe it in The Goblet of Fire. Because The Truth Potion is so powerful that three drops would have divulged anyone's innermost secrets.
It wouldn't necessarily have solved the problem.
The Ministry's main line was that Harry was lying about Voldemort's return. If he had repeated his story under the influence of Veritaserum, the Ministry could (and likely would) have responded in one of two ways.
1) Harry had somehow duped the Veritaserum's effects. How credible this is would depend on whether it was the Ministry's own Veritaserum, and whether people think fifteen-year-old Harry is really that powerful.
2) Harry believes it, but the story is still false. Essentially, someone-- perhaps one of his friends?-- went to a lot of trouble to make The Boy Who Lived think Voldemort had returned. This is the more credible Ministry line of the two (and it's important to keep in mind in the real world, also-- making a false claim isn't necessarily lying, if the person honestly believes it). Granted, this might have drastically altered Umbridge's punishments toward Harry; but the official Ministry story of Voldemort Definitely Hasn't Returned would have remained unchanged.
Confundus Charm can be used to make people believe in things. Veritaserum test would not be able to detect it.
“Black had bewitched them, I saw it immediately. A Confundus Charm, to judge by their behaviour. They seemed to think there was a possibility he was innocent. They weren’t responsible for their actions. On the other hand, their interference might have permitted Black to escape … they obviously thought they were going to catch Black single-handed”
Harry Potter And The Prisoner of Azkaban, Chapter 21, Hermione’s Secret
It wouldn't have made any difference when this was Fudge's response to the fact that Barty Crouch Jr had provided a full confession under the influence of Veritaserum:
'As Minerva and Severus have doubtless told you,' said Dumbledore, 'we heard Barty Crouch confess. Under the influence of Veritaserum, he told us how he was smuggled out of Azkaban, and how Voldemort - learning of his continued existence from Bertha Jorkins - went to free him from his father, and used him to capture Harry. The plan worked, I tell you. Crouch has helped Voldemort to return.'
'See here, Dumbledore,' said Fudge, and Harry was astounded to see a slight smile dawning on his face, 'you - you can't seriously believe that. You-Know-Who - back? Come now, come now ... certainly Crouch may have believed himself to be acting upon You-Know-Who's orders - but to take the word of a lunatic like that, Dumbledore ...'
Harry Potter and the Goblet of Fire - p.611 - Bloomsbury - Chapter 36, The Parting of the Ways
Sure it helps that Barty Crouch had been Kissed by then, so neither Fudge nor anyone else could hear more, but Fudge had already gone into full denial mode. All he needs to say is that Harry may have believed it, or even been fed lies by Dumbledore, and Veritaserum makes no difference. This is especially true when we consider that Fudge believed Harry was mentally unhinged because of Rita Skeeter's articles:
'You are - er - prepared to take Harry's word on this, are you, Dumbledore?'
Fudge still had that strange smile on his face. Once again, he glanced at Harry before answering. 'You are prepared to believe that Lord Voldemort has returned, on the word of a lunatic murderer, and a boy who ... well ...'
Fudge shot Harry another look, and Harry suddenly understood.
'You've been reading Rita Skeeter, Mr Fudge,' he said quietly.
'And if I have?' he said, looking at Dumbledore. 'If I have discovered that you've been keeping certain facts about the boy very quiet? A Parselmouth, eh? And having funny turns all over the place -'
'I assume that you are referring to the pains Harry has been experiencing in his scar?' said Dumbledore coolly.
'You admit that he has been having these pains, then?' said Fudge quickly. 'Headaches? Nightmares? Possibly - hallucinations?'
Harry Potter and the Goblet of Fire - pp.611-2 - Bloomsbury - Chapter 36, The Parting of the Ways