If you can heat it above boiling temperature with a gas torch it's not the water line!
But a better test is to bang on it and listen to the water line and gas line and see which has the loudest echo of the banging (just put your ear (or a stethoscope) against the side of the gas line and the water line.)
Try searching for where your water and your gas are metered outside.
Imagine a line extending perpendicularly from the house so that it would intersect the water meter. There's a good chance the actual water line will follow that imagined line.
Next, look at the pipe coming off the gas meter and entering the house. Is it the same size as the pipe you've found inside? Does it happen to have the same tape wrap? If you imagined a straight line extending off the ends of the section of pipe you can see, would it extend toward the gas meter location?
If all that isn't any help, give us more detail. What's the age of the house? Of what material is the pipe (copper, silvery or black steel, plastic)?