My old foreman used to tap the mold with a screwdriver handle, listening for a dull thud that meant a hidden crack. Pat424 is right about the light, but sound catches what light misses sometimes.
In our shop, we inspected every mold with a bright light held at an angle. Shadows make even hairline cracks obvious from the side. Also, avoid pouring into a cold mold, the temperature difference stresses the sand. If cracks keep showing up, your sand mix probably has too little moisture or binder.
Seriously, you guys check every single mold with a light like that? That's some next level patience right there. I'd probably get halfway through the first one before saying "good enough" and just pouring. But I guess that's why my hobby projects look like junk and your shop stuff doesn't. So you're basically giving each mold a full medical exam before it gets any metal. Makes sense, but man, that sounds like a slow way to start the day.