Testing for psoriasis?

Hi I have been diagnosed with seb derm and rosacea at the end of 2015. A dermatologist had put me on Rozex, protopic and lymecycline for 6 months. Mid year my course ended and I was discharged by the dermatologist. I had a second flare up after this course ended. So I'm back on a 6 month course of lymecycline again.

However at the back of my mind I am still in doubt if it's been diagnosed right. What tests are used in diagnoising psoriasis?

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The doctor usually takes a biopsy and will test the tissue sample. Did he do that?

Also with Rosacea which my sister has, Minocycline has really helped her for the last 10 years.