Pop a wheel off.
Bend a wire, get a pointy stick, or cobble together something to point at the rotor.
Turn the rotor by hand.

Does it wobble back and forth?

If so, check both sides. Compare the runout.

If it's got a bump on one part of the rotor it was likely a hot spot that was pushing the lathe cutter away from the rotor.
I've seen it happen on my lathe and it's far sturdier than any brake lathe at a repair shop.

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon