You should be able to tell by the color, and connections used.

Gas

"Black pipe" is commonly used for natural gas, and is dark grey/black.

All connections will be threaded.

Water

Water lines come in a variety of materials, some of the most common are.

Copper

This will be um... copper, in color.

While there are other ways to join copper pipes, the most common is solder.

Galvanized

Galvanized pipes will be a light grey.

And use threaded connections similar to black pipe.

Plastics

You might also find various types of plastic-ish water pipes. These can come in a variety of colors. Red, blue, white, black, etc.

These may use compression type connections, crimp connections, or various other type or connectors.

Still can't decide?

Try using your other senses.

Touch the pipe.

If it's warm to the touch, it's likely hot water (may not be warm unless hot water was recently used). Hold the pipe and have a helper turn a nearby tap on/off quickly, to see if you can feel vibrations. Try running the water for a bit, then touch the pipe to see if the temperature changes. If the pipe gets colder/hotter, it's likely a water line.

Listen to the pipe.

Put your ear to the pipe, and again have a helper turn on a nearby tap. If you hear the water clearly in the pipe, you found yourself a water line. (make sure the pipe is not in contact with any other pipes when you do this, since sound could be transferred to the other pipe). This is not the most accurate method, but it can sometimes work.

Taste the pipe.

This won't help you at all. I just pictured people licking pipes in my head, and thought it was funny. You could tell your helper to do it, and then laugh at them when they do it. But it's not going to help you figure out what the pipe is.

By shutting off the water main, there is no way for expanded hot water to be relieved by backing into the main. This causes over-pressure and either the pressure relief valve is letting some water out (hopefully into the pan) or the high pressure water is opening a flaw in the tank or a through-hole, such as a heating element, spigot, or water connection.

The water temperature is not held perfectly steady. It varies up and down by perhaps 20 °F (10 °C) or more. (Test this by turning the thermostat up and down and notice how far it goes past the current temperature before it clicks.) If no hot water has been called for awhile, the temperature drifts down to the lower range and remains there most of the time. Odds are that closing the main valve occurs at a lower water temperature. The valve closure traps and holds municipal water pressure and then the water heater's next heating cycle builds pressure on top of that.