It is kinda hard to explain. Once you have looked at literally tons of carpets, you start to be able to spot the crosses.

Coastals (M. s. mcdowelli) are the largest of the carpet pythons growing to 10 ft. plus. They are basically a scrub and coastal environment python. They tend to have more earthtoned colors.
Jungles (M. s. cheynei) have sometimes been described as a rainforest coastal carpet. They are a bit smaller and range from 4.5 ft to 7.5 ft. These are typically black and yellow carpets.