I've been trying to figure out the exact definition of "A Clean Break" -- meaning that the milk has set, and is ready to cut.
The best thing I've found is to a (clean!) finger, poke it into the curds about 1" deep, at a diagonal, and then lift your finger straight up. If the curds "break" as you lift through the top, then you're good to go. If they just kind of ooze around your finger, they're not strong enough yet and you should wait a bit longer.
This page on Dr. Fankhauser's site has some good information about getting a clean break.