What kind of test are actually carried out on cosmetics labled
"dermatologically tested"?

Hello Nicola,
I answered your other question titled "cosmetics": 

http://answers.google.com/answers/main?cmd=threadview&id=231569

I suggest you close this question since the content is the same.

"Dermatologist-tested" can mean whatever the manufacturer wants it to
mean.  The FDA currently has no standards to regulate this (its
attempt at regulating the terms "hypoallergenic" and "natural" were
overturned in court), and thus there is no guarantee that a product
won't cause a reaction.  The article below says that while the terms
are used in marketing of products, dermatologists say that medically
they carry very little meaning.

You may try to contact the manufacturer of a specific product to see
what tests they conduct, but you are not likely to find much.

Read the following article for more information.  
http://www.fda.gov/fdac/features/1998/398_cosm.html

Hope this helps.