How scientists can be confident that human embryo tail is really a tail?
We can be sure it's a tail because it is an extension of the vertebrae, and we define "tail" as an extension of the vertebrae, among other things. 


  tail
  
  tāl/
  
  noun: tail; plural noun: tails
  
  
  the hindmost part of an animal, especially when prolonged beyond the rest of the body, such as the flexible extension of the backbone in a vertebrate, the feathers at the hind end of a bird, or a terminal appendage in an insect.
  


Emphasis mine. Source: www.oxforddictionaries.com