What is another word for an identifier?

Edit: Give a better explanation of my problem.

I am writing a computer program. I like to make analogies within my code to help me express the intent when I read it months down the line and it all looks like gibberish (code easily becomes gibberish if you aren't careful).

I am interested to find another word for someone/something who/that identifies something. The best I can come up with is "an identifier".

There is currently a theme of a police investigation throughout my code. There is a Detective and a Suspect; the Detective interrogates the Suspect (a web page).

I want a word to identify a bit of my application that, given a response (from a web server), after some investigation, identifies the implications of or meaning expressed by that response.

So far I have named the code the "identifier" because it identifies the meaning of the response.

The word must express the intent of identification, because that is the purpose of the code. Although witness is the right word in the context of the analogy it is not the word that I am looking for because the issue for me is expressing the purpose of the code.

Bonus points if you identify a better analogy. (When I reach 15.)

Solution 1:

Classifier - Identifying means deciding which of a collection of predetermined classes the particular thing belongs to.

Solution 2:

Not exactly what you asked, but a taxonomist is concerned with the classification of things, especially biological.

An appraiser might give something a value, and might identify that thing in the process.

Or you could make up your own word; how about determinatrix?

Edit: In the context you give in comments, a witness would be the person choosing from a line-up.

Solution 3:

In the sense of "the expert identified the plant" then there isn't really a word.

There may be some obscure legal terms for eg. the witness who proves someones identity on a contract