Primary Indicator: Is it an Electric Water Heater?
Primary Indicator: Open the tank. Is the water level going over the overflow tube?
Primary Indicator: Did plunging it fail to clear the stop?
Primary Indicator: Is the pressure low at every single faucet?
Primary Indicator: When you flush the toilet, does water come up in the shower/tub?
Primary Indicator: Can you isolate the shut-off valve for that fixture?
Primary Indicator: Does the water get cold only when someone flushes?
Primary Indicator: Is it leaking from the vacuum breaker (cap on top)?
Primary Indicator: Lift the float switch manually. Does the pump turn on?
Primary Indicator: Does it bang when the washing machine stops?