If your doctor suspects that you may have FLD he will most likely suggest you see a Gastroenterologist to perform some tests.

This will usually occur if your blood test shows increased levels of the liver enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST).

You may be given an Ultrasound that is called a Fibroscan. If you suspect that you may have FLD it is suggested you see a Gastroenterologist to perform this test. The test is sort of like an ultrasound as the technician squirts liquid on your right abdomen and then uses the machine to thump your side approximately 10 times to get a reading of the thickness of your livers walls. 

The results give you what is called a CAP score the is a measurement of the fatty change in your liver.  The cap score is measured in decibels per meter. (db/m). It ranges from 100 to 400 db/m. 

You will also get a Fibrosis result which is a measurement of the amount of scarring in your liver(aka fibrosis) This measures scarring by measuring the "stiffness" of your liver. The result is measured in kilopascals (kPa). Normally between 2 and 6 kPa. The highest possible result is 75 kPa. 

Below is a sample image of  a Fibroscan result

Other possible tests may include a cat scan or an MRI of your abdomen.

Finally a liver biopsy may be required in order to verify the extent of your liver damage. 

The National Institutes of Health offers a page outlining tests for Fatty Liver disease here.