Assuming you spent it all on one AMEX card it's real easy to check.
1) Login to your account, go to recent activity,
2) Under 'time period' select 'year to date'
3) Under 'narrow results' click 'other filters', click on 'transportation' and deselect everything besides fuel.
Click Continue
At the bottom of the screen it will total all of your fuel transactions. Obviously if you often shop at gas station mini marts for things other than gas that will artificially inflate your total.
Post:
1) $ amount spent
2) Make Model & Year of car
3) Approx mileage driven
4) State you usually fill up in