I believe it is diagnosed via a blood sample.
There is no specific test to diagnose it, unfortunately, usually tests are done to rule out other diseases and they come to the conclusion that it must be lupus. I believe there is one test they do, on the white blood cells I think, that gives them a pretty good idea that it is, but mainly it's just symptoms and the lack of any other diagnosis.
There is no definitive test.
Lupus is diagnosed
1. Symptoms
2. History
3. Variety of labs
4. When other similar disorders are ruled out