I used to work in Victoria Wine for 2 years so I got to handle 1 or 2 notes. Here are some things I learned about telling fakes. I may have forgotten a couple of things so /msg me for additions.

Telling the forgery

When you first start handling notes, you may find that checking for all these things takes a while. Don't worry, after a short while, you get used to it and do it automatically.

  1. Feel:
    This is perhaps the biggest give-away as to whether you're holding a fake. It takes a little while to start to notice but when you know the difference, you spot it immediately! In short, you will notice that some (bad) fakes feel just like normal paper that you write on,others might be "waxy". This is because real notes are made from cotton paper which is made and processed differently. It's a good idea to rub your fingers down the note when you first get it in your hand to get a proper feel of it.
    The reason I don't like handling US Dollar bills is because they all feel fake to me!
    Be warned, some fakes DO use cotton paper so try some of the other ones as well.
  2. Ink:
    Bank notes have special types of ink. You may have noticed, when you run your fingers over some of the writing, it will feel slightly raised. This writing is the "Bank of England" script at the top of the note. Also, on the left of the Queen's head, you will find a shape that is almost entirly coloured in the main colour for that note. If you get a blank piece of white paper, and rub this shape against it, you will see that the colour rubs off. This tends to work best with newer notes.
  3. General Colour:
    Some fakes you will find tend to be the same colour (monochrome) or the paper is slightly tainted.
  4. Watermark:
    The watermark is where an image has been impressed into the paper while it was wet (hence the name). To look for this, hold it in front of a bright light and you should see a shadow of the Queens head. Beware, some of the more clever fakes try to fool you by having a very faint one printed in the space. Some might even have a real watermark of their own, but these are rarely as good quality as the real ones. If in doubt, compare them to a real note.
  5. Forgery Strip:
    This is a strip of foil woven into the paper itself. If you hold it up to the light and you should see an unbroken shadow that runs right down the note. Some fakes might have some silvery stuff on the note but I've yet to see a fake that has a proper foil strip.
  6. Serial number:
    Sometimes you might get a warning to look out for certain serial numbers. As a general rule, these numbers are stamped onto the paper so they should leave slight indentations. This is not always obvious though, and tends to show up better with new notes.
  7. The person:
    Sometimes, the first warning is the person that gives you the note. If they look nervous are worried when giving you the note, then it might be a good idea to check it out.

Non Bank of England notes
In Scotland, Northern Ireland and a few other places (I can't remember where) certain banks print their own money (see legal tender and Scottish Banknotes for more information about this situation)! The designs printed on the notes varies and so can confuse someone without much experience.
BUT:

  1. They are printed on the same paper, with the same dimensions.
  2. They use the same security devices.
  3. They are not legal tender just because it says Pounds Sterling, most places will take them if they they pass the above forgery checks. (Strange bloody situation if you ask me)
I know some of this might sound obvious but I have actually seen people accept the most blatant forged note just because it was Scottish and therefore different. Some places even refuse to accept non Bank of England notes to stop any confusion. Mostly this is unnecessary if people know what they're doing.

There are also some things specific to certain denominations:
New £5, £10 and £20 - the new notes carry a hologram on the front as well as a red and green number (for that denomonation) that is clearly visible under UV light and micro lettering under the portrait of the queen.
£50 - The £50 note has a silver foil circle with a red rose in the middle on the right of the Queen's portrait.

sources:
experience
bariau
gkAndy
http://www.bankofengland.co.uk/banknotes/factnote.pdf

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