How does a shadow clone identify itself as shadow clone?
At the beginning of the series, Naruto's Shadow Clones did confuse and fight amongst themselves about who's the original one (before realising that all they had to do was to release the jutsu to figure it out).

I'm guessing that as you get more proficient in the technique, both you and the clones begin to recognize who they are.


  In Naruto's most recent case, the original has Gudoudama behind him, while the clones do not.