GPL Violation. What can you do ?

Say I (or you) develop some piece of software.
Decide to release it and choose to do this under the GPL: .

Then someone fetches the code, makes some minor modifications and release's it without source, charging money, basicly violating the GPL.

Thinking we are in our right we decide to sue the ones that breach the licence.

However the deal goes sour and the person/company you were sueing countersues you, saying you stole the software from them somehow.

How can you prove you wrote the code first ?
What does count as proof in court that you wrote the code.
On a local machine files could easily be forged. (simpe example: bios, touch util).On remote machine it would ne harder but ' a remote machine is a local machine somehwre ' .
How can you proof you were the first one to write the code ?
The GPL and the GPL-faq could not give me a satisfying answer.

In IRC someone ( i think it was franky ) told me: "you should print the source-code and mail it to yourself, leave it unopened and its valid, because of the marker on the stamp" ;D.

As a small developer you prolly aint got no money to pay for them lawyers.

Im trying to focus the discussion on:
'what stands in court'