I use first method to check the notes:
Running your finger across the front of the note you should feel raised print across the words "Bank of England".
? Holding the note up to the light and checking the watermark. You should see an image of the Queen in the clear oval area in the middle of the note. Carefully check ?20 notes which, contrary to popular belief, are the most widely counterfeited notes, not the ?50. A bright ?20 should also show up in the watermark on a ?20 note and a bright ?50 on a new ?50 note.
? Look for the metallic thread running through every genuine note. It appears as silver dashes on the the back of ?5, ?10, ?20 and old ?50 notes. If you hold a note up to the light, this thread should show up as as a continuous dark line. The thread is fully embedded in new ?50 notes so there are no silver dashes, but held up to the light it should still show up as a dark line.
http://www. theguardian.com/money/2013/oct/19/how-spot-fake-banknote