My Dexter was born a cotton coat and stayed a cotton coat and his 3 siblings were silky coats. I could tell what my boys were when I got them at 9 weeks old (I have 2 brothers from the same litter). My other boy has a silky coat. Dexter started matting pretty bad around the 9 month mark I noticed. He had his first grooming at 11months old. I remember that much. I tried to brush him daily to keep up with the mats but it got to be too much work, so he got and has kept a puppy cut all his life. He'll never have a long coat, its so much work, he mats sooo easily! The mats brush out easily though, if you have a pup that does not mind being brushed and you have an hour a day to do his whole body! haha I didnt have either there!
I don't know if this is true or not for all pups I did hear it on here once awhile ago and its true for my boys. If your pup has a cotton coat his/her eyes will glow red... if he/she has a silky coat the eyes will glow green.
first pic is puppy pic, dex is closer
second pic is this summer - 3 years old - dex is further (dex is the cotton coat)