They are usually difficult to pick as hatchlings and get easier and easier over their first year or so, though some individuals will even surprise you a little in their second year or so.
If you want one which is definitely going to turn out really nicely with very little peppering, you'll be paying extra for it unless you buy it as a hatchling and get very lucky. The background colour is usually somewhat evident right out of the egg, but you really need to examen them closely in the flesh. Most of them hatch out with no pattern, but the heaviest patterned individuals will show it before they hatch.
If you buy from random people who are breeding from drab parents, you'll probably get dull babies. If you buy from people breeding from beautiful parents, you're more likely to get better looking animals. You can either buy hatchlings from people working with pretty animals and roll those dice, or buy their holdbacks for significantly more money, or buy from people working with normal ones and get what you're likely to get.
I was one of the first people to start working with them (I actually held and photographed the original marble back when she was still the only one that existed and before they had the name 'marbled'). It has been a lot of fun and in over quarter if a century of working with snakes it's right up at the top of the list of my favourite projects. The quality noticeably improves every year. Literally more than half of the marbles we produced this season would have been non negotiable keepers just 2 years ago, and it has been painful selling a lot of them!
I'll post a few pictures when I'm at the computer.
Incidentally, I have marbles available at the moment if you're interested.