They won't ask you for any money for the tax disc, that's how you'll know!
Look here-http://www.vcacarfueldata.org.uk/search/search .asp- and type in your car- if it produces less than 100gCO2/km, then it's band A, but don't expect it to be there: I can't think of one band A car other than that horrible Gwizz thing.