The smell of sulfur or rotten eggs, a hissing or whistling sound near a gas line, a white cloud or dust cloud near a gas line, bubbles in water.
Unusually high water bill, the sound of running water, wet or damp floors, foul odors, wall cracks.
Rusty deposits or a metallic odor coming out with hot water can signal either rusty pipes or rust inside the water heater. If corrosion is eating way the metal form the inside, leaks are inevitable.