A liver enzyme test is just a simple blood test. If your doctor ran a series of blood tests before putting you on the Lipitor, it may have already been included in the series. Look for AST and ALT on your copy of the lab results. If a doctor knows these tests are needed, they normally just order them at the same time as the lipid blood profile. I know in years past, my doctors have included those tests just as part of my routine annual physical. If you didn't get a copy of your lab results, you might call the doctor's office and ask them to mail you a copy.
Someone else please tell me if there are different lab tests which might have been used instead. I'm no health professional and am only familiar with the tests the doctor has ordered for me. I assumed that since statins have the potential for damaging the liver just as does niacin, that doctors would normally order some kind of test to make sure the liver is in good condition before they start. Maybe I'm wrong.