I assume you mean Cuckoo Marans.
Marans have white feet, and if they are from the French lines are feathered. British Marans are clean legged, I believe. Barred Rocks should have yellow legs, but often have some dark on them, especially from hatcheries.
The Cuckoo pattern is a little muddier than the more defined black and white stripes on a Barred Rock.
Egg color is also different with Marans laying a dark chocolate brown egg, and Barred Rocks laying a light brown egg.