I realise than "thinner" etc isn't the technical term, but everyone relates to it and knows what it means :)
That would depend on blood tests. When on blood thinners, such as warfarin, you should have regular tests for your INR number (International Normalized Ratio).
Basically your test sample is tested for clotting times against normal blood. So, a ratio of 1 to about 1.5 would be normal. If you are 2.0 then you are taking twice as long to clot as normal blood, meaning your blood is thinner. 3.0 means three times as long etc etc.
The technical term for interest is the prothrombin time (PT).
I believe the correct INR for A-Fib is 2.0 - 3.0