how do you evaluate the dermal toxicity of modern drug?

During preclinical tox testing, mice are the most adjectives.
According to the FDA guidance on woundcare products (from the CDER website):
When preclinical studies or previous clinical experience suggest that a topical product might induce clinically significant dermatitis, irritancy or sensitivity testing surrounded by normal volunteers is recommended prior to trials contained by patients, since superimposed dermatitis is deleterious to wounds. The need for routine conducting tests of the final formulation depends on the product, and sponsors are encouraged to discuss dermal toxicity trialling with the appropriate Center up to that time initiating the studies.
Here is an exapmple of a Percutaneous Absorption Study for a topical drug product..
Toxicity/Irritation Tests:
Acute Toxicity (LDn)
14-Day Dermal Toxicity and Irritation in Mice
7-Day Dermal Irritation within Guinea Pigs
Primary Skin Irritation in Guinea Pigs
Primary Eye Irritation contained by Rabbits
13-Week Dermal Toxicity and Irritation Test in Rabbits
91-Day Dermal Irritation and Toxicity Study
rub it on your skin :) sorry, bleak joke.