how can you tell that the melanizer gene is missing from some of your birds?
It is the columbian gene which is missing from some of mine.
First, some of mine would have to lack one or both columbian genes because they had a black mottled in their background. Black mottled does not usually have colombian genes.
Second by sight. If a male has one columbian gene one cannot easily tell by sight, but might with offspring depending upon whether the female carried one or two.
If a male has no columbian genes it will have a black mottled breast (I have not ascertained whether it might have any colour at all) most likely, if it didn't have melanisers, it would have mille fleur colour in the places where males usually have gold, but would be black mottled in parts where males are usully black.
A pullet without columbian is easier. With one columbian gene there is quite a bit of black marking on the back often in the cushion area. With no columbian it would look the colour usual for the e-allele only with mottling, most often with a black line, possibly with more black if melanised.
I always intend to get photos, it is much easier to show rather than describe. Unfortunately my skills as a photographer are so poor that it is rather offputting.