Wet or soggy areas may indicate a leak in your backyard. If it hasn't rained recently and you have an unusual amount of standing water in your yard, you most likely have a leak.  Also, keep an eye out for unexplained increases in your water bill. It is most likely the result of a hidden leak.
Look around your yard for any dying vegetation. If your plants or grass are dying in one area of your yard, it could be because of an underground leak affecting the roots.
After turning off all water sources in your home, you can also check the water meter. It may indicate a leak if it continues to run or changes.