It can be almost impossible to know what the mix is for most of them. You can sometimes see Silkie or Cochin or Game Bird in many mixes. Once you move away from those three it's almost impossible to know without seeing the flock they came from.
In your research, if something doesn't sound right to you, ask about it. There is a lot of incorrect information out there. Things that hurt the birds and make the keeping of the feathered ones less than pleasant for the humans.