I wanted to find the volume of water in an olympic-size swimming pool. In one source I got the value of 660,253.09 gallons of water. Knowing that there are at least two values of a gallon, roughly 3.8 liters and 4.5 liters, I looked up the names and values of these two types of gallons. The approximate 4.5 liter gallon is known as the imperial gallon, and the approximate 3.8 liter gallon is known as the US gallon, or more specifically the US liquid gallon.
In the Wikipedia article on the gallon the first photo in the introduction shows a can of gasoline which is said is for use in the US and Canada, and is labeled in imperial gallons. (Error: It shows "imperial gallons" for Canada and "US gallons" for the USA.
Further down in the article it shows a photo of an American fuel pump, with prices shown in what I think are US liquid gallons.
So it appears that both imperial and US liquid gallons are used in the US.
My question is, how can I know which type of gallon unit is used when reading about gallons, for example, with the information about the swimming pool?