backdate for purpose of deception

What is the name of your state? UK

All,
Mine is a general question:

Some one wrote official letter to me and backdated it for the purpose of deception. I am 100 % that the letter was written after 20/09/2004 but in his
letter he put 20/08/2004 instead.

How can you prove the letter was not written on the date put in the letter?

Thanks,

Bambumbile

[cbg]

US law only.

Nor do I understand how this is a legal question to begin with.