The Marans will lay a darker egg. You should be able to tell a slight difference in barring in both. If you have a Marans and the breeder/establishment bred to the French Standard (I do believe) they should have feathering on the shanks. They will also have different leg color as illistrated
A Utility Barred Rock Hen (NOT MINE)
A Cuckoo Marans Hen (NOT MINE)
A Utiltiy Type Barred Rock Rooster (NOT MINE)
A Cuckoo Marans Rooster (NOT MINE)
Barred Rock Egg
Barred Rock Chicks 5 Days Old (NOT MINE)
Marans Chicks (NOT MINE)
But remember the Cuckoo Marans Eggs tend to be lighter than other vairieties. At least this is from what I read.
Barred Rocks should have yellow legs, Cuckoo Marans should have pearly pink. Barred Rocks should have clean legs, Cuckoo Marans should have feathered (but hatchery quality ones are clean) Barred Rocks should have a good barring, Cuckoo Marans should be darker and more smudged in barring (although in hatchery quality, both are rather smudged) Barred Rocks should have a nice brick-like body, Cuckoo Marans should have a chesty, almost upright body (again, in hatcheries they're both the same though)
Another thing of note, hatchery-quality Cuckoo Marans will and can lay eggs just as pale as your run of the mill "brown" egg.