I've only seen hairline cracks. It's not always easy to spot them, a close look with a clean rim is helpful.

This is the most recent rim crack, on a HED Ardennes. The wheel still rode fine, and the spoke still has tension, until the crack gets longer. I retired it.

At first, hairline cracks a few mm long. Then the spoke nipple starts pulling through the rim as the cracks get longer. It can be a slow process.
Even with a spoke pulled through, the wheel should still get you home, with some out of true wobble.