Cuckoo Marans chicks gender

[AusHen]

I just purchased 14 unsexed Cuckoo Marans chicks. 12 are a day old and 2 are a week. I have read a "smudge" on the head means cockerel and a defined spot means pullets. Is this true or just means more likely this and that but not exactly relable?
Here are two of the day olds that are noticeably different

 

[Reawsphant01]

yes, I hatched out one that had a dot on its head and it was girl so the smudges are roos and the dots are girls

 

[NatJ]

I have read a "smudge" on the head means cockerel and a defined spot means pullets. Is this true or just means more likely this and that but not exactly reliable?

Good question! I just went off and did some google searching... I was able to find more information on sexing Barred Rocks than on Cuckoo Marans, but the color genetics are the same.

Here's something published in 1941 that includes lots of photos of Barred Rock chick heads, and discussion about details of it:
https://archive.org/stream/sightsexingbarre00cana#page/4/mode/1up
The author says on page 10 that "over 80%" of chicks can be correctly sexed by their headspots. So that's fairly good, but not perfect. And as I look at the photos in that guide, my eyes just aren't seeing some of the differences. Must be a skill that takes practice!