Recent cars tell you how many miles left to refill and how much to put in (in vehicle info). Not sure about older ones. But why do you need to know? Adblue is widely available now and the easiest rule is to wait until the car tells you it's low with a warning message on the dash, then add NO MORE than 10 litres.
If you have the Pro-Services app on your phone or computer you can check this level remotely under 'Vehicle Health Report' section, It tells you the current mileage range to next adblu fill required and current capacity required to fill it.