Domestic longhair - unless a cat is a first cross of a very recognisable breed (and many breeds are only a few generations from their origins, unlike most dog breeds, and so are little more than crossbreeds themselves), it is hard to tell what cross a cat may be.
In fact even two well established dog breeds can produce a cross that doesn't look much like either parent and once you get past first crosses, appearances can be very deceiving indeed.
A first cross Maine Coon or Norwegian Forest Cat would be notable by its size, they are huge, not just 'a bit of a big cat' but properly massive, huge feet, loooooong body, big head. Simply having long hair with a pronounced ruff or main, and tufty ears does not denote a Maine Coon mix.
I would not worry about breed unless you suspect Exotic/Oriental genetics - in that case it's usually pretty obvious. Those fall into the 'chaos cat' category of cats, and their behaviour can be hard to live with for those not expecting it or not prepared for it.
Most people want a cat from the 'decorative' end of the scale, who are easy to live with, friendly and a bit playful from time to time.
Anyway this cat looks lovely, a Jellicle Cat, very smart and glamorous!