How to tell if a pipe is pressurized

I need to tap into a line that is going to a hose bib, so that I can add another hose bib in another location. The problem is that in the place where I'm gonna dig to get to the pipe, there are actually 3 or 4 pipes running together in that trench. Only one is pressurized, that is, going to the existing hose bib. The others are different sprinkler zones. Is there a way to determine which pipe to tap into? Or do I have to just start cutting until I find the right one?
Thanks.

Assuming the water is cold, run the water out the hose bib and feel for which pipe is the coldest.

you may also feel the vibration of the water through the pipe.