Trace Copper Pipe

[Shubox56]

I own a townhouse with a closet dedicated to utilities -- furnace and hot water heater. It's a tight space with the water main, cutoff valve, and meter located behind the hot water heater. To cut off water to the house, I have to reach around the heater --- PITA! Anyway, there's 2 copper pipes (a branch) after the cutoff which extend through the slab, and though uncertain, 1 of the pipes may lead to an outside wall which feeds the outdoor spigot (GUESS WORK). Is there a means of determining if one of the pipes is feeding the outdoor spigot? And equally as important, if it's dedicated to ONLY the outdoor spigot? If it is dedicated, I will remove the hot water heater and hire a plumber to install a cut-off valve to eliminate frozen pipes at the slab near the spigot. I did install a cut-off valve in the outside wall, but it does nothing to protect the pipe at the slab (a much bigger concern). The picture shows the 2 pipes that I'm talking about. They are right next to the wall.