They have a lot of sensors on everything, plus they'd be filming from a lot of angles, and the additional benefit of ground testing is that they can look at the

They have a lot of sensors on everything, plus they'd be filming from a lot of angles, and the additional benefit of ground testing is that they can look at the debris. You can piece together where the explosion happened based on the charring and where the parts broke and how. If I recall correctly, the Dragon explosion was especially interesting because the reaction was previously unknown. So, it wasn't just a design flaw that allowed NTO to leak into a helium line, it was also a new discovery that titanium can react with NTO under high pressure and ignite.