I remember reading in the old Marans thread some tips for sexing chicks. Sadly, I can't remember who posted them or find the page again!
One tip was comb size - and some Marans roos do show themselves quite early this way. However, I've also has some that don't show much color or size in the comb until 5 or 6 weeks.
The other was foot color. Chicks with dark feet, including black toes = pullets. Chicks with some lighter color on the toes = roos.
I read this right after my last set of chicks. There were only six in this group, but it correctly IDed the 2 roos and 4 pullets. I'd be interested to see if it works for any of you!