As far as I know, other than a full blood count and liver function tests.. there's no way in knowing if you have a fatty liver.. it's really about your diet. If you know you eat a lot of fatty foods and drink a lot of alcohol then it's very likely you have a fatty liver. The pre op diet is just to eliminate the chance of it as it makes the surgery more difficult if you have a fatty, brittle or enlarged liver.
When I had my surgery, my surgeon told me my liver wasn't fatty but enlarged which made it difficult to move out of the way to get to my stomach he actually nicked it because it was that big. My surgeon didn't mind if I did the pre op or not as he's very experienced but I definitely cut down on the bad food and stuck to salad and shakes 2 weeks before the operation, the last thing you want is to be woken up from being out only to be told they couldn't do the surgery.