Finding out if a web user is using Tor

Question

Recently I read an article on the Tor developer site about removing responses for certain calls to Tor programming built into Mozilla's code. By that token, and depending on what they remove, would it not be possible for a site to attempt to use commands removed in the package just to determine if the user were in fact using a Tor browser and not a more standard Mozilla distribution?

Update:

The data I refer to (found in my notes) appears to show concern for revealing information about the web browser being Firefox-based and what operating system is being used. The provided answer seems to describe reasoning why this is however not important:

https://trac.torproject.org/projects/tor/ticket/13016

A snippet from that site:  

In Firefox25, Mozilla added a couple scary media queries
(-moz-os-version and -moz-osx-font-smoothing).
I think we should get rid of these, as well as most/all of the prefixed 
media queries in 
​https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries#-moz-os-version.
Either just disable them, or make them lie.

Answer 1

There are two different questions lurking here.

When you use Tor to connect to a website, the IP address of the Tor exit node will, of necessity, be visible to the website that is being contacted. In addition, the IP addresses of Tor exit nodes are public, so it is easy for a website to learn that the user is using Tor, which is quite a different matter from learning where the user is located. Websites like Wikipedia regularly determine whether a user is using Tor. Tor users are not prevented from viewing Wikipedia, but the user will be prevented from editing pages in a certain way. Other sites, such as the Australian telephone directory, regularly block all Tor users from using the site.

At the user end of the Tor connection is the question of whether or not you are trying to hide your use of Tor from your ISP. For most Tor users, this is not an issue, but it is an issue in those countries that censor internet in some way and want to prevent users from using Tor to circumvent the censorship. If this is what you are interested in, then search for articles on the Tor project blog relating to the circumvention of censorship.