Until now, there was no way of knowing....
1. Normal wear and tear
When you install a toilet in a unit, you are adding a few gallons of water to your building's usage. Every gallon weighs 8.32 pounds, so when a few gallons are added, it's very heavy. Plus, water is corrosive. Slowly but surely, the toilet tanks let more water flush into the bowls, and that causes a ton of water waste.
2. Toilets don't flush as advertised
Many low-flush toilets have been retrofitted with parts that cause them to use much more water than they were designed to handle. 55% of the 300 million toilets in the US have been upgraded to a model that uses 1.6 gallons or less, but independent research has shown that the majority of these toilets use 2 gallons per flush or more — every time they are flushed.