Or there's the genetic test that recently became available.It will be alot of test breeding to root out the heterozygous birds. But doable.
https://orders.iqbirdtesting.com/product/blue-egg-gene-with-feathers-sample/
It's a bit expensive, but probably less expensive than test mating, because you don't have to raise a bunch of pullets to laying age just to see what genes their parent carries.