By bizn_fun 
Check the holographic window
On genuine polymer £20 notes there is a large transparent window. Inside it you should see a portrait of King Charles III or Queen Elizabeth II. Around the edge of the window the value of the note and the words Bank of England are printed twice. The window should be clear and sharply defined, not cloudy or blurred.
Tilt the foil
On the front of the note there is a silver foil patch with a hologram. When you tilt the note, the words change, for example between “Twenty” and “Pounds”. If the image does not shift cleanly when you move it, treat the note with suspicion.
Feel the raised print
Genuine notes have raised print you can feel with your fingers. The words Bank of England and the number value in the bottom right corner should have a textured finish. If the surface feels completely smooth, that is a red flag.
Do not rely on counterfeit pens
You can buy so called counterfeit detection pens online, but they were designed for older paper notes. Since the UK switched to polymer notes, they are no longer reliable. Checking the built in security features is far more effective.
Compare with a genuine note
If you are unsure, compare the suspect note with one you know is genuine. Look at the colours, the clarity of the print and the feel. Small differences can be easier to spot side by side.
Consider safer payment methods
If possible, ask for a bank transfer instead of cash. A transfer leaves a digital trail and removes the risk of being left out of pocket with fake notes. If you do accept cash, count and check each note carefully before handing over the item.