There are several indications that there might be a leak in your plumbing system. You might have noticed a sudden dip in your water pressure, for instance, or a sudden spike in your monthly water bill. Neither of these signs offers conclusive evidence, but it's important that you know there's a leak so that you can get it repaired as quickly as possible. Fortunately, there are DIY water leak detection tests that can determine if you actually have a leak.
If you have a water meter, the first thing to do is to make sure nobody's using the water. Shut everything off. Don't take a shower, do laundry, wash dishes, or flush the toilet. Next, take a look at the meter itself. You'll see either a needle or a triangle. If it's moving, you've got a leak.