How do you know if your hot water heater is frozen?

If it's just the pipes leading in and out of the unit that are frozen, you should check it. This will happen a lot. The spot where it's ice cold can be found by touching the water heater.Check if the water heater itself is frozen or if it is only tubes that enter and exit the unit. This will often be the case. You can check by simply touching the water heater to find where the ice cools.