How does Lupus become diagnosed?

Lupus is not an easy disease to diagnose and your doctor need to rule out other possible disease scenarios before confirming the diagnosis of lupus.  Since there is not a lot of information regarding exactly why lupus happens it is hard to pinpoint the exact clinical manifestations that might be present. The symptoms of lupus often mimic other illnesses and sometimes some symptoms will be present and not others.

If a doctor was considering the possible diagnosis of lupus they would look for signs and symptoms of inflammation which are generally pain, heat, and redness, swelling, and loss of function in a certain place in the body. Inflammation can occur in the inside and outside of the body. Inside the body the lining around the heart and kidney disorders causing excessive protein in the urine are symptoms of inflammation inside the body that a person with lupus would have. Inflammation can occur on the outside of the body as well it will manifest as a malar or butterfly rash, raised red patches, and photo-sensitivity. There also may be neurological manifestations such as seizures or psychosis. Other manifestations that can occur are hematologic or blood disorders like anemia, low white blood cell count or low platelet count.

            Since Lupus is such a hard disease to pinpoint the doctor will go through numerous laboratory tests to gain a full picture of the forming of the illness. Even if you have some of the symptoms no single laboratory test can determine lupus. A test result one time may be positive and negative another time and different laboratories may produce different results. If someone presents with multiple criteria simultaneously then a physician might diagnose a patient with lupus. Most often though these manifestations happen over time and the diagnosis may not be as obvious. Setting up an appointment with a rheumatologist may be necessary to either diagnose or rule out the option of lupus. 

References: 
http://www.lupus.org/answers/entry/diagnosing-lupus
http://www.lupusresearchinstitute.org/lupus-facts/lupus-diagnosis