Summarize this article:

Your doctor can determine if you have psoriasis by looking at your skin, scalp, and nails. They may check for any red, scaly patches or inflamed areas on your skin. They may also look at your nails to check if they are pitted, discolored, or ridged. Your doctor may ask you about your symptoms to help diagnose your issue. They may also consider your medical history to see if you are prone to skin conditions or have sensitive skin. Your doctor may take a small sample of your skin, called a biopsy to confirm the diagnosis. A biopsy can also help your doctor determine the type of psoriasis you have and rule out other disorders. A skin biopsy is often done at the doctor’s office. An anesthetic may be applied to the area so you do not feel the removal of the skin sample. The results of the biopsy are often ready within a week. Your doctor should be able to use the results of the skin biopsy and the physical exam to make sure you have psoriasis and not other skin conditions. There are several skin conditions that look similar to psoriasis, including:  Seborrheic dermatitis: This skin condition causes your skin to appear greasy, itchy, scaly, and red. You may notice seborrheic dermatitis on your face, upper chest, and back. Lichen planus: This skin condition often appears as red, itchy bumps or lesions on your arms and legs. Ringworm: This skin condition is caused by a fungal infection. You may develop red, scaly rashes in a ring or circle. Pityriasis rosea: This skin condition appears as one large spot on your chest, abdomen, or back. It may then spread, forming the shape of drooping pine tree branches.

Summary:
Let your doctor do a physical exam of your skin. Get a skin biopsy. Allow your doctor to rule out other skin conditions.