There was a post on here a couple of days ago from a guy in the photo trade who has seen a few fakes, including Manfrotto tripods and B+W filters. He said they were pretty much indistinguishable unless compared side by side.
Apparently you can tell a fake B+W by the light weight, as all B+Ws have brass mounts, not alloy. You can also tell if a filter is coated or not - single coating shows reflections off the glass as blue, and multi-coating as geeen (generally). Non coated filters will always reflect back white. Goes to check those I already have.
I doubt that anybody would bother to coat a fake filter of any kind, so that's a clue, but the only way to ensure the genuine article is to buy from a respected retailer that gets stock through offical channels. It's easy enough to check who the official importers are in each country, although if they don't come from them they can still be genuine of course, via a different route. Just that if they do, then they are genuine for sure.
