Only way I know how to do that is to cross to a single comb white or brown egg layer and confirm each resulting hen lays blue or green eggs in the F1 generation. Would be a bit harder with a flock of hens though, easier to do with a rooster.
That's exactly what i did. I had 3 easter eggers. 1 dominant white (all babies were white), 1 with muffs/beard, 1 clean faced. The whites were mixed blue and brown eggs. She got sold. Thankfully the other 2 were ok. I can ID the clean faced ones to the clean faced hens and the muffed ones to the muffed hen.