If you google "car tax check", go to the website and enter your registration number, you will see the car details.
If you make a note of the CO2 emissions, then google "vehicle tax rates", go to the appropriate table, and you will see the rate for your car.
As said above, if your car has high emissions, you pay more!
That's why I'm sticking with my old Astra diesel for the time being (£155 a year).
My son's Euro 6 Peugeot 2008 diesel has a zero rate of tax, and goes almost as well as my Astra (but is much more comfortable)! Guess what I will be looking for when my car gives up the ghost!