A few of the signs that it was a cross bred are these things:
White or pearl colored or blue earlobes, supposed to be RED
White or slate legs, or green (if it was crossed with EE of some sort)
Lays white, light brown or simply light brown egg instead of a terra cotta color (can range from light...
Uh oh! Not a good thing! LOL!
Watch out for the color and ear lobs and legs and feather patterns....it can tell you if they have been crossed with something else like OEGB's.
Did you say bantam flock, not LF?
The Welsummer bantams will NOT be found in hatcheries, only from a breeder and they are pretty strict about that. However they will not lay as dark as their LF cousins. More like a light tan in color.
What are you looking for? Egg production? Silkies...