Diagnosing an autoimmune disease involves identifying the antibodies your body is producing:

Autoantibody tests: any of several tests that look for specific antibodies to your own tissues.
Antinuclear antibody tests: a type of autoantibody test that looks for antinuclear antibodies, which attack the nuclei of cells in your body.
Complete blood count: measures the numbers of red and white cells in your blood; when your immune system is actively fighting something, these numbers will vary from the normal.
C-reactive protein (CRP): elevated CRP is an indication of inflammation throughout your body erythrocyte sedimentation rate: this test indirectly measures how much inflammation is in your body.
https://www.creative-biolabs.com/drug-discovery/diagnostics/autoimmune.htm