How does a shadow clone identify itself as shadow clone? All shadow clones have the same memories as the original (from birth until the Jutsu cast time). Which means all the shadow clones also should think of themselves as the original. 

If an action needs to be done by only the original, all the shadow clones also would try to do the same. Because they all think that they are the original.

Is it valid? Do shadow clones know that they are just clones?
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.