If you've a post March 2001 vehicle you can check its CO
2 level by doing a "Vehicle Enquiry"
here. All you need is the registration number and make.
My 2005 Td4 (manual) is currently in band 'F', and produces 205g/Km, so will be in band 'K'.
This means my tax now, next year and in 2010 will be £210, £215 & £245 respectively.
New tax rates are
here.
HTH :)
Edited to correct a typo