If an average Joe and Jane Smoe think they discovered a treatment that works for them but it's not currently used by conventional medicine and they want to validate it for effectiveness and safety, they can ask an experienced doctor, in case of psoriasis a dermatologist, for an opinion. The doctor can then judge if the treatment deserves further investigations and find experts who would be willing to do them. In the US, it's the Food and Drug Administration's (FDA) Center for Drug Evaluation and Research (CDER) who makes the final validation and approval of a treatment:
Drug companies seeking to sell a drug in the United States must first
test it. The company then sends CDER the evidence from these tests to
prove the drug is safe and effective for its intended use. A team of
CDER physicians, statisticians, chemists, pharmacologists, and other
scientists reviews the company's data and proposed labeling. If this
independent and unbiased review establishes that a drug's health
benefits outweigh its known risks, the drug is approved for sale.
The best way to evaluate the effectiveness of treatment is by randomized double blind placebo controlled studies.
Randomized means that a group of participants is randomly divided into 2 groups: the main group receieves the studied treatment and the control group a placebo (the treatment with no effect).
Double-blinded means that during the study neither the researchers nor the participants know who receives the real treatment and who a placebo.
The results of individual studies can be further evaluated by systematic reviews and meta-analyses, which are considered the highest level of evidence.