The answer to your *updated* title, how to demand that the car match the manufacturer's rated fuel economy - due to the factors listed towards the end, I don't believe this is feasible.
The answer to your immediate question; how to check before driving if the car matches a certain fuel economy rating; can be done by the following:
Turn on the car, check the on board trip computer. Almost all modern cars have a fuel economy readout which tells you what was the fuel economy on the last trip. However, if someone has reset the trip computer, this value may not be accurate.
Carry with you a OBD adapter and a cellphone app. Plug it into the OBD port and read the last fuel economy reading.
Practically speaking - car fuel economy ratings are like laptop or cellphone battery estimates - they are done in controlled conditions which rarely apply to real life use. Cars sometimes match, sometimes exceed, but often struggle to match the ratings.
The figures on the sticker are to be used as a guideline only - they are not hard numbers.
Saudi Arabia has introduced fuel economy rating stickers on new vehicles sold, which are also used in other countries in the GCC. Here is what it looks like:
As you can see, the number is more of a guideline of how good or poor is the fuel economy of the vehicle. The actual figure is as a result of controlled testing.
I don't have recent experience with Enterprise, but what I can tell you is I have rented the exact same car (in fact, ironically, it was even the same bland white color) from two different reputable international franchise rental agents from the airport in Bahrain. Each car performed differently when it came to fuel efficiency. There are many reasons for this:
- Maintenance - a poorly maintained engine is not as efficient
- The grade of fuel used, if different than what the car is rated for
- The gearing / how you drive. If you constantly use the manual pedals, "sports mode", etc. this all affects the fuel economy.
- Tire pressure
- Driving with the windows up or down, AC on or off, etc.
- Ambient temperature and altitude
I am not sure what can be done to reliably predict how a car will perform - short of actually driving it.