look at the cylinders and where they mate to the head. If the cylinders outer jacket looks bulged out and like they are wider than the head, then that means there was freezing and they have expanded. When that hapen's there is usually a crack that forms and leak's follow.
Sometimes they just bulge and are just fine like that. But inside you can see the outer jacket has cracked in a few places and has separated near the bottom of the cylinder casting.
But since you haven't had any running issues or leak's your lucky