You can also monitor your water usage, even by the hour, on the DEP website (http://www.nyc.gov/html/dep/html/customer_services/amr_learn_more.shtml). If you follow the suggestions phillybubby made, and watch the website as you turn things back on, you may be able to target where the problem is. I know that my water usage always spikes on laundry day.
It’s also worth it to contact customer service, and verify that no one else is tapping into your line or worse. Call 718-595-7000.