Sarcoidosis is a systemic disease that is manifested by granulomatous infiltration of multiple organs sites, with the most common being the lungs, followed by the lymph nodes. Other sites of involvement include the liver, skin and eyes, with the heart, CNS and kidneys bringing up the rear.
The most common way someone is diagnosed with sarcoidosis is via a chest x-ray, followed by physical exam. Salivary gland enlargement and tear duct changes are physical findings that can clue you in to the diagnosis.
On the skin, the manifestations are so protean that some people call it the "great imitator", much like they used to call syphilis the same thing. Commonly, one can see papules, particularly on the face, often in the perinasal region. One can also see erythema nodosum, which is nonspecific, of course, but can be strongly associated with significant pulmonary involvement.
The best diagnostic test is a biopsy which shows noncaseating granulomas. Other lab tests include an elevation of serum ACE levels, as well as hypercalcemia.
As always, in cases like this, your best friend is a biopsy.