How do you know if a kid is telling the truth about needing to go to the bathroom or faking it to get out of class?

Today was the first ay of summer school. There was a boy who seemed like a troublemaker so I refused his requests. I thought he was faking it when he squirmed around. He ended up wetting his pants and I feel awful. How do you know if they're being truthful or not?
Update: This isn't preschool btw. The boy is 13 or 14.
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