I see a purplish spot on mh chin when I wake up...

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how to diagnose  a dermatological disease?

From your complain I understand that there is abnormal skin spot that might be due to dermatological disease or might be normal, so you need to be examined by doctor.
The differentails for this spot might be due to:

• Tinia versicolor
• Blood spot
• Acne
• Local inflammation
• Wart
• Other cause

 
In general you need to be examined to diagnose the cause from above mentioned. If the cause is due to fungal infection then the treatment is by using antifungal treatment. Also if the cause is due to acne then you can use fusidic acid cream. As you see without clear examination it is difficult to reach diagnosis.
Feel free to message me anytime here if you have further questions.
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