In the case of the bomb, you know he's telling the truth if the bomb is where he says.
For terrorists, you ask things like "How can we find person X?", "Where do you keep your stuff?", "How do we decode the messages you've been sending?", "What is the password to this iPhone?" -- things where you can look and see if his info checks out.
There's other methods. Ask the subject for lots of details. "Where did you meet X? Oh, it was a restaurant? What did you order? The restaurant was closed that day, you just earned more waterboarding".
Are you confident you can keep a false story straight, under stress? Remember all the false details you said after days of no sleep? Eventually, you will decide that only the truth will save you from bad experiences.
Problem with that kind of validation is you’re putting lives on the line. Which brings up the question of how many lives you’re willing to bet on the guy telling the truth vs whatever he needs to say to make the pain stop.
Unfortunately in the world of torture the truth doesn’t necessarily make things stop. The truth is messy, things aren’t remembered fully, things aren’t known fully, the guy can’t tell you what he doesn’t know. He’s actually much more likely to be able to give you a “whole” story that’s made up, he “knows” all the parts of that, he’s making them up. If you’re torturing a guy and he tells you the truth and you don’t believe him just once you’ve pushed him into fabrication land, he has no choice he needs the pain to stop.