Usually people can tell a genuine piece by asking yourself: Is it Soft-paste or Hard-paste porcelain?
It says here (Millers pot & porc marks), hard paste porcelain was made from 1768 onwards.
Since the mark on the shoe dates before 1753, it should be made from Soft-paste porcelain. It does say however that there are also soft-paste copies but not as many as hard paste copies.
I have no idea how to tell the difference between Soft-paste and hard-paste porcelain.
Hard paste porcelain has a very glassy quality to it, and the glaze is bonded very tightly to the body. So if you have a chip, it will almost look like opaque glass, and you won't be able to see the glaze sitting on the outside of the porcelain. It is quite cold to the touch as well. Most continental porcelain is hard paste.
Soft paste porcelain is much warmer to the touch, and any chip will look as you would expect porcelain to look, the glaze sits on the surface, and doesn't bond to the body in the same way as hard paste, so you will see this glassy surface over the porcelain on a chip. It may be a creamier colour. Much English porcelain will be soft paste.
It is one of those things that is much easier to tell when you have two pieces sitting alongside each other.