The best way to tell real milk glass from fake milk glass is to look at the milk glass closely.
If the milk glass is real milk glass the real milk glass should be slightly translucent and if you take a light to the milk glass the light should easily shine through the real milk glass.
if the light does not shine through the milk glass then it is likely not real milk glass.
Some milk glass may also have markings on the milk glass that says it's real milk glass but sometimes they do not.
Also if your milk glass has a Hobnail pattern, and the bottom is marked with a Fenton mark, you can be more certain you do indeed have real milk glass.
There's newer milk glass and older milk glass and the older the milk glass the more likely the milk glass is real genuine milk glass.