I saw this done once on a house in Grand Prairie, so Arlington might be pretty close to the same regs.

Lone Star (Yeah, it was that long ago) tapped the line out by the street and isolated the line to the house from the street with a stopple. They then injected nitrogen until they had reached three times the working pressure which meant taking it to 60 psi. The line leaked so they sleeved it with a piece of that plastic pipe and tested it again. That time it held.

If you hook up to the existing outlet you will see line pressure and nothing more. If you tried to pressure the line you would need a semi load of N2 since you would be pressuring up the entire system as you would have no way of shutting off the flow to the mains.

A better way to find a leak would be to either rent a hydrocarbon detector and run it up and down the line. These thing are meant to pick up hydrocarbons which are lighter than air and will make their way to the surface.

You could also contact the local gas utility who might come out and check with their hydrocarbon tester for free but if they do find a leak they will want to repair it and since it is on your property they will then send you the bill.

I can understand you would want to know this, but if you are never going to use the line then why are you concerned?