I wrote a few thoughts on this subject here:
AuthenticityThe major problem with judging authenticity is not having any official Playmobil guide or catalogue for reference. A lot of the time my own judgement is based on instinct. That's not to say that I couldn't also make a mistake, but I like to think that this is less likely after collecting them for so many years.
I check the keychain listings regularly (used to be daily, but now I'd say 3-4 times per week), so I get a feel for what is readily available in the marketplace. If I'm doubtful about authenticity and am considering bidding on an item, I usually write to the seller first, asking them more about where they obtained the figure. If it is an older one (like a fig from a retired set), I will sometimes ask whether it has any other identifying marks like a logo on the back. Unfortunately, not all keychains have this now, which is a bit of a shame as this was a very useful way of spotting the real thing.
In the case of the seller above, when I first saw one of the unusual keychains they were offering, I was almost duped into believing it was an official one too. But when I checked their other listings, I noticed they were selling
several supposedly rare keychains I'd NEVER seen elsewhere. This was the warning sign for me - for a seller to come across one or two "new" keychains might be plausible, but to find a whole bunch of rare ones and no "common" ones at all is highly unlikely.
All I can say is if you're ever in doubt, contact the seller for more info. Sometimes they might be able to tell you the origin of the keychain, othertimes not. Failing that, ask me and I'll give you my gut instinct on it.
I have ALL the common keychains on my site. The only ones remaining for me to get are the rarer ones now.