The easiest way of finding out is to wait until the light comes on, and then immediately go to the filling station. The difference between how much you can put in and the 37 litres which is the size of the tank will tell you.
The most accurate way of measuring is to wait until the light comes on and then stop the car.
Then you should syphon the remaining fuel out of the car and place it in a measuring jug.
Then use the moulded plastic numbers on the outside of the jug to see what you have removed. You may have to fill the jug more than once depending on how big it is so have a few old juice bottles to hand to store the removed fuel.
If you need to use the measuring jug again for food use then remember to use a lot of fairy liquid and give it a good clean out.