How can I monitor my mains water use?
Water meter calibration is something your water company can do for you, or you can hire an independent company to do it. I saw one water company charge $70 for calibration.

When water meters lose accuracy with age it is usually in your favor: large-scale calibration efforts have been profitable for water companies because replacing inaccurate meters usually leads to more revenues for them.

Before doing anything, test for leaks by turning off all faucets and appliances and verify that the meter stops. You can also learn a lot by watching the meter as you use water in different ways. (You will be surprised to see how much watering the lawn is costing you!)

You could try a rough DIY calibration by filling a large vessel of known volume, such as a kiddie pool, but you may need several samples to get an accurate estimate.