Top up your oil make sure you are on level ground and check the level of oil with the dipstick once it is at the top then do 1,000 miles checking steadly to make sure if it is burning lots that you dont let it get to a low level.
The other way is drain the oil and fill up to max then when you have done the mileage drain again and measure.
All this will tell you is the amount of oil used over a set distance and more the 1ltr then you must have a issue.
Hope this helps, but have you been driving the car harder or different recently?