Fortunately, in the field I normally collect (Transition Towns and Micronations), there is very little call for people to fake notes. Many of them have quite sophisticated security measures and a lot are worth little more than their face value. No one (yet) has got round to forging the Totnes first issue, but I know the way to check those!
However, I have recently seen some Black Sheep Bank notes in an American auction, which had unusual backs on them. They went for an amazing price, and luckily I didn't see them until after the hammer had fallen, so I wasn't tempted to bid on them. A couple of weeks ago, they appeared again on eBay with quite high starting bids. Rapidly, I placed a holding bid. Then I thought to myself, I wonder whether they are geniune? The backs in particular were unlike anything I had ever seen before. Knowing the original auction house was very reputable, I hadn't queried their listing, but now I wanted to check.
One thing with Black Sheep notes that have been through the Treasury, is the blue, oval seal on each note. On the specimen above this is on the right and shows TWO PENCE. In the late 60s (and for decades before) any promisory note had to be stamped by the Treasury to be valid. Richard Williams, Mr Black Sheep, had long and convoluted tussles with the Treasury, but that's another story. Many of his notes were sealed. You can always tell a genuine 'sheep' by feeling the seal. If the note is raised, then it's more likely to be right. If it's flat and smooth under your fingers, it's definitely a copy.
I messaged the seller on eBay, and he told me the blue seal was flat. COPY! As a good trader, he immediately withdrew them from sale and we struck a deal, my knowing I was buying copies. Well, they arrived. I can 100% confirm these notes are not genuine. Not only was the seal photocopied, but the paper quality would make Richard Williams turn in his grave!
If you are offered Black Sheep notes, that's always the first thing to check. Just a little tip! If in doubt, you can always ask us. If you want to see 100% guaranteed notes, check our dropbox link - the fakes are at the end of the listing with a prefix starting with a z and copy in their file name.