How can a muslim be sure if his mark on the forehead is from prayer (prostration), or from sle or psoriasis?

SLE vrs Psoriasis: You can do blood work like ANA to determine if you have lupus and HLA CW6 to see if you have psoriasis. A skin biopsy would also tell the difference.

Agree with all : I believe all options given for laboratory testing are valid. Most definitive way of telling what the lesion is would be to perform a punch biopsy. Other tactic might be to pull a photo of yourself from as far back as you can where there is a good shot of the forehead/facial area. Get a magnifying glass or scan the pic and look at it under high resolution in the computer. Is the lesion there?