If its not happened already fake 20p error coins will find there way into the collectable market hopefully this advice will help.
Your 20 pence coin should have no date on either side
It should measure
21.4 mm in diametre
And weigh 5 grams if using digital scales and it reads 4.9 to 5.1 then this is acceptable
If you dont have access to digital scales then visit your local post office and use theres?
Most coin sellers on ebay know there coins and have access to digital scales so before you buy or bid ask them the weight if there genuine then they will be more than happy to do this for you..
As a member of coin clubs I have not heard of any fakes yet although some idiots have tried grinding off the date!!!! I have another ebay guide on the error 20p coin you could look at for more information
Enjoy your ebaying