I have two black cables comming out of my wall that was connect to an
old water heater. They have no markings on them tell me which is hot
and neutral. There is no ground wire. The voltage across the two black
wires it 240V. The water heater I am installing is 240V. So I am good
there. On the new water heater there is a red cable and a black cable.
My question is: Can I connect either black cable comming out of the
wall to the red cable on the hot water heater? I know there should be a
white and a black comming out of the wall, but there isn't. If I have
to use a specific Black cable comming from the wall, how do I tell
which one?

Thanks