How would I know if my engine cracked on my saab 900SE?

I've been using water over the summer because my waterpump has been broke for a while. It was cold the other day and the water froze and wouldn't start (of coarse). It defrosted for now and starts up, but now overheat due to a leak that spills from under the engine. Hopefully it's a freeze plug and not my engine cracked.