Just a small leak from a toilet can double your water usage in a month! Those leaks will cost you money, and waste valuable resources.
To check your toilet for leaks, put a few drops of green or blue food coloring in your toilet tank. Let sit overnight. If your toilet bowl is green or blue the next day, you've got a leak. Replacing the leaking parts in the tank costs about $10 and 30 min of your time.
Also, run your dishwasher and washing machine for a full cycle (full obviously). Then shut off the main water to your house. Find your water meter and take a peek. If the meter is spinning, you have a leak somewhere.
All in all, you'll spend about an hour of your time checking for these issues, but you can save so much money in the long-run!