What's the noun for diagnoser? Here's the word you're looking for.
diagnosis
(medicine) The identification of the nature and cause of an illness.
The identification of the nature and cause of something (of any nature).
(taxonomy) A written description of a species or other taxon serving to distinguish that species from all others. Especially, a description written in Latin and published.
“Each of these techniques provides a distinct diagnostic of a given species, such as total production rate, spatial distribution, temporal variance, temperature, or velocity distribution.”