It's important to check for more than one security feature.
- Check the feel of the paper and the raised print across the words "Bank of England"
- Hold up to the light to check the watermark. An image of the Queen should be visible in the clear oval area in the middle of the note
- Look for the metallic thread running through the note. It should appear as a continuous straight line.
You'll also find more detailed information on the Bank of England booklet "take a closer look"