Tor users can be identified as Tor users one way or the other. Now Tails users are subset of Tor users. Can this subset be identified as Tails users? Or this subset can not be distinguished from other Tor users. The point being, anonymity is in numbers, and total Tor users are probably orders of magnitude larger than Tails users. If Tails user in any way become distinguishable from other Tor users, The probability that he can be de-anonymized will increase by orders of magnitude.
There is two different levels on which you can be identified as a Tor or Tails user. It is not clear from your question which you means, so I will answer both.
The website server you visit knows if you are using Tor or Tails
Tails users can be identified as such by its browser fingerprint. For example, Tails comes with AdBlock Plus, and Tor Browser Bundle (TBB) doesn't. The configuration in the browser also differs somewhat in other regards.
Since most TBB users download ads, but most Tails users doesn't, that allows to guess whether you are a TBB user or Tails users. The exact list of ads Tails blocks is also fixed for a certain Tails release, so a TBB user that have installed AdBlock Plus will probably not have the same list, which makes the guessing even easier.
Tails does try to keep its fingerprint as close as possible to the Tor Browsers however, which of course makes it somewhat hard to fingerprint. It will probably keep improving further in the future, for example: Tails have discusses whether or not to remove AdBlock Plus in a future Tails release.
See section "3.11 Fingerprint" in Tails design document.
Your ISP or network admin knows if you are using Tor or Tails
Tails does only send Tor traffic, whereas another operating system with TBB on will also sends non-Tor traffic (e.g. syncing clocks, checking for system updates, etc). If your computer only sends Tor traffic, it is probably Tails.