Irrigation leaks are the worst way that you can waste water as an irrigation user. They are repairable, and sometimes even preventable. Repeat after me, "I WILL fix all leaks in my system". The best way to stop leaks is to make sure that the system has been installed correctly, but for an existing system all you can do is manage it. Inspecting a system takes time and math. Your system can be perfect on one day and have a leak the next. That is why you need to move to a proactive mindset over a reactive one.
We can all give some horror stories about bad installations. I have dealt with pipe that was primed but not glued, gasketed mainlines that did not have thrust blocks, and individual systems that were designed so poorly that we found over and under pressurized zones next to each other. While these problems can be managed after the fact, they are much cheaper to fix before installation. Look hard at any designs that you are given. If you are using city water or a VFD pump, check to make sure that there are no zones over the max GPM that you can receive. Make sure that you have pipe sizes that can handle the flow. On a non-VFD pump, make sure that you have similar GPM requirements for your zones. Balance them as much as possible to stay in the sweet spot of the pump curve.
There is a simple test that we use to determine if we have any irrigation leaks on the property. The only way this can be done is with a flow meter. If you do not have one installed, make that priority number 1. It is hard to conserve what you cannot measure. Other tools needed are a pitot gauge, and the spec sheets for the heads you have. Do not assume that the system works properly. Many leaks are hard to find.
Pick a good zone to start with. To properly inspect your system, you will have to go through all zones. Turn on the water and use a pitot gauge to check the pressure for each head on the zone. If they are pop-up heads, use an inline gauge. Make a note of the nozzle size and with the pressure find the gpm that each individual head is using. After you add up the total for the zone, compare that to the flow meter readings that you have. They should be very close. Most of ours are within .2 gallons. Sometimes you will find pressure readings that are off the charts. Make notes on these as this is probably an area that you can make improvements on the design.
Repeat this process for a couple more zones. If you notice that all of the zones are reading what they should be, congratulations on not having a mainline leak. If they are all off by a similar gpm or percentage, I would start looking for a main leak. If only 1 is off, it is time to start looking for a leak after the valve.
We have been able to check a 28 acre, 94 zone system in a day using this process. Once you get the hang of it, it moves quickly. If your system does not learn the zone flows for you, make notes on the flows for future reference. We make monthly visual checks on every zone and head, but are able to use our learned flows to check for leaks every day on every zone. In fact, the smart controller we have will let me know about them without me spending one additional minute. If you don’t have a system that will alert you, try to check your flow meters at least once a month, if not once every other week. This will ensure that you are not wasting water through leaks.