Maine Coon cats have a very distinct body style and some telltale characteristics that are unique to them. They have long 40 inches or more bodies that are rectangular shaped and exceptionally large paws that can sometimes have an extra toe. Their paws are fur-covered and the ears usually have hair at the tips.
Confusingly, all of the above characteristics can also be found on Maine Coons that are mixed with other breeds. Frankly, there is no real way to tell if your cat is a mix because the difference is already so subtle between a purebred and a mixed breed.
A purebred will obviously have all of the physical traits that are exclusive to a Maine Coon, and a mixed breed will have many of the traits and sometimes all of the traits. A typical sign that your coonie is a mixed breed is that they are not quite as large as a purebred coon.
Size Variance
The Maine Coon is a very large cat. They can weigh up to 25 lbs not because they are overweight, but because their body mass is spread evenly throughout their body. Their tails can measure up to 16 inches in length. Typically if they are of mixed lineage they will not grow to the massive lengths that a purebred does.
A purebred coon cat will grow for up to 5 years before it reaches its full maturity cats that are of mixed breeding usually reach their full potential at 2-3 years of age.
Other Variances
Since Coons come in a rainbow of colors and patterns, relying on the color or pattern to determine full versus half breeding is not a reliable method but their coat may offer other clues as to whether your Maine Coon cat is of pure of mixed breeding.
Purebred coons have ruffs of hair that are discernible around their necks. They are built for the cold Maine winters and their coats look like they are built for the cold. They have thick undercoats and light fluffy coats that are usually medium to long-haired.
Their tails are always very fluffy and full. They do shed their winter coats but even with a lighter spring coat on they are pretty fluffy cats. Sometimes when they are bred with other breeds they lose some of their fluff and some of the telltale unique qualities like the neck ruff of fur.