That is the major problem with breeding “possible hets”, it’s a total crapshoot/ long odds lottery and the math is not in your favor. It’s best to just breed 100% hets, or breed a “possible het” because you want the co-dominant genes anyway regardless of wether the recessive gene is present. That way if both parents are hets, it’s a surprise bonus, even if highly unlikely.
Or if you are really intent on testing there genes, you have to breed a possible het to a visual recessive to see what happens, but I personally would not waste the energy when I can just get a 100% het or visual to start with.