Marans should have white soles on their feet, and I'm pretty sure Cochins should have yellow soles feet.
Foot color on chicks can seem to change when they are very young, but it usually settles down to the correct adult color within a few weeks.
(Of course "purebred" chicks with the wrong foot color do exist sometimes, but with most chicks from hatcheries the foot color is correct more often than it is wrong, so it is still helpful for figuring out what breed you have.)
Cochins are supposed to have heavily feathered feet, and Marans would typically have sparse feathering, but those feet look in-between to me. So I wouldn't want to decide either way, based just on that.
Cochins generally grow up to be much more fluffy than Marans, and Cochins lay eggs that are a much lighter brown than Marans eggs, but of course you have to wait for the chick to grow before you can check those traits.