Water Heater

The easiest thing to do is go outside and lift the pressure relief valve on the front of the water heater. If water comes out, then your tank if filled. If not, then the tank is being bypassed. If the tank is being bypassed, then you need to look at the back side of the water heater. You'll see one or three valves (depending on type of bypass) that prevent water from flowing into and out of the tank (bypassed) or allow it to enter the tank (not bypassed).
There should be something around to indicate what position the valves need to be in for normal operation or bypassed operation.