Is there a way to identify which pipe is which in a group of pipes?

I need to connect a new pipe to an existing buried PVC pipe of a particular zone in my irrigation system. At the point where I need to connect, the pipes for two zones make 90 degree turns, so I can dig back a bit for flex, cut off the elbow and replace it with a tee to connect the new pipe. Only problem is that I cannot tell which pipe is on the zone I need to connect to. I know I have a 50% chance of getting it right by just cutting off one elbow and turning on the desired zone to see if water shoots out. If it is the right pipe, I just add the tee. If it is the wrong pipe, I replace that elbow and then remove and replace the elbow of the other pipe with the tee.

Before I do that, is there any trick to identifying which pipe is which? In the past when I came upon a piece of underground pipe and tried to tell which zone it was on I had tried running each zone and listening to the pipe with a stethoscope to see if I could hear the water flow, but I couldn't. Are there any other methods I can try that doesn't involve specialty equipment? Maybe having someone tap on the end of one of the sprayheads to see if I can hear it in the related pipe?

You might detect small temperature changes that occur with running water. If you have one of those indoor/outdoor electronic thermometers it might be sensitive enough to pick this up. You'd have to mount the temp. sensor temporarily onto the pipe. The pipe with running water will probably be colder than the other.

If there is a section of metal pipe ahead of the manifold, you could heat it with a torch, then turn on the zone and feel for the temperature change when the hot water arrived at the hole you have dug.

And if it's half-inch pipe @ 1 GPM it'd take one minute per 100' of pipe for the leading edge of the blob of hot water to get there.
1" pipe, 1 GPM, 100', 4 minutes.

But PVC has 2000x more resistance to heat flow than copper so you might need to leave the torch on for a while.

This trick might actually work.

Okay, here is what I did. First, one of the zones had shrubbery risers which are directly piped (no flex pipe) so I had my wife tap the nozzle while I felt the pipes. Though it was subtle, I could feel the tapping a bit more in one pipe than the other. I wanted to verify the test, so I tried checking the temperature of the pipes with an infrared thermometer after the sun had warmed them to about 84 degrees. I then turned on the shrubbery zone and the same pipe as before dropped to 75 degrees. I could actually feel the pipe being a bit cooler. So, Thatguy, the water temperature idea worked out fine. Armed with the results of both tests, I cut into the other pipe and it was the one I wanted.

Thanks for reporting back.
I guess water-filled pipes transmit tapping pretty efficiently, even if they are made of plastic.
You da' man!

I had the same issue once. I took a tomatoe stake and rested it against each of the exposed irrigation pipes. Once I put my ear to the wood stake, the rushing water was very easy to hear.