Primary sources of wasted water include leaky toilets and inefficient irrigation systems (i.e. overclocked cycles, broken sprinkler heads, damaged irrigation lines, etc.). Oftentimes, these leaks may occur without any noticeable signs.
For example, a "stealthy" toilet leak could be caused by a defective flapper valve. To determine if this is the case, place a few drops of food coloring or a dye tablet in the tank, and then wait about 10-15 minutes. If the color seeps into the bowl, the flapper valve is not sealing properly and should be repaired.
If your irrigation system is leaking, you may notice the sprinklers malfunctioning, or your lawn may seem especially saturated, water-logged, or "squishy" in some areas. You will need to replace any broken parts or hire a qualified plumber/landscaper to inspect the system.
For additional resources, please refer to this guide to detecting leaks.