In fact, it is up to the plainant/prosecution to prove beyond doubts that the defendant is guilty.
I don't know in this case how they can do it without cooperation of Russia.
They will to prove intent. They will have to prove that the missile was russian property.
And they will have to have the jurisdiction....
The counsel will have just to ask the proof (independently certified) that the missile was fired from Russia. Russia has to prove nothing.
That is how the law works.