I've seen loads with M&S stickers on the base.
Anther dead give away is when the glass is clear with a film applied to the inside. This often gets scratched.
I've picked a few up being impressed by the shear size 20" or so high. I think I've seen those with 'Made in China' on the base. These were cased glass.
As I have not handled the real deal, I leave the rest alone until I'm more clued up.