I have a gas line to my house for the furnace but need to know how to tell whether I can hook up a gas range in the kitchen without having a gas line installed to the spot where the range goes.
Just as a combination answer bringing together several very salient comments:
The space for your stove is gas-ready if and only if there is a gas line, generally either copper or steel and with NPT threads, protruding from the wall or floor behind your existing stove, with a shut-off valve installed. A line that is capped at the end without a shutoff valve indicates that you have gas service at the right location and so you can install a gas stove, but the shutoff is required by code so that in the event of a leak you can shut off the gas at the source instead of calling and waiting for the gas company to shut off your whole house service (and then having to re-light pilot lights etc when the gas is turned back on).
In addition, the burning of fossil fuels indoors generally requires that the space has adequate exterior ventilation to the outside. 9 times out of ten, this means a vent hood (or combination microwave/vent hood) that exhausts to the outside. This vent removes the combustion by-products (carbon monoxide and carbon dioxide being the two most harmful) so they cannot build up in the kitchen and pose a hazard.
To connect the gas supply line to a gas range, a flexible hose is used. This hose is traditionally yellow in color (and in fact that may be required by code), and while flexible, it is extremely durable, because a failure of this line can be catastrophic. The line is connected to the range and to the supply line, and then a thick soap-based mixture is painted on to all the connections before the shut-off is opened. If the soap mixture bubbles up, there's a leak that must be addressed. As is said in the trade, "no bubbles, no troubles".
Generally, I personally would recommend leaving this to a pro. There are things you probably do not know about the materials involved that can create a deadly situation; for instance, did you know that a copper gas line cannot contact anything made of a different metal? This is because different metals will react galvanically with each other, to the general detriment of both metals. A copper gas line touching a heating duct, or a steel pipe, or even being attached to structure with improper anchors, will rust out over time and create a gas leak that can destroy a house in the blink of an eye.
Generally, HVAC technicians are licensed for NG work (because they regularly deal with gas furnaces), and will generally be happy to come out and hook up your stove as well (and install a proper external vent line for your range hood). Plumbers are also commonly licensed for NG work because many of the tools and materials are similar. There are also natural gas contractors, which usually handle installation of gas service for new homes or renovations, but can do it all from the gas main under the street to the connection to the range. If you call someone out to work on your home, make sure they're licensed for natural gas installation; a general handyman service or the like will probably not be licensed for any of the work they do, including gas hookups.