First, find the water meter that feeds your home. There should be a small analog water movement indicator that looks like a small clock with only one hand. Turn on a water faucet in your home and that hand should be spinning like gang busters. With all faucets turned off, that hand should not move at all. Watch the hand for about a minute or two and see if it slowly moves. If it moves, then you have a slow leak somewhere. If it moves fast, you have a major leak.
For comparison, we had a slow leak under our concrete slab. Looking at our water meter, that hand moved 1/2 revolution in 1min 18sec. After I corrected the problem, the hand didn't move at all in 15 minutes.