[Brad54]

For the guys saying if the engine never fired, there's no exhaust gases so it couldn't get into the coolant system through a crack, etc. etc.
No, but there IS compression in the cylinders, and if there's a cracked or warped head, or blown head gasket, that compression wold absolutely still get into the coolant passages, exhaust gas or no.

Check your oil and see if there's any coolant in it.
Quicker than a compression test, do a leak-down test. This pressurizes the coolant system, and it'll tell you if there's any coolant going into the cylinders (which is the problem you have, but the other direction)