To be technical, you can't prove that he lied to Congress. He said he hadn't taken them, which could be true. For all we know, he took them for the first time two months ago.
Now, do I THINK he just started? No. But in order to prove he lied, you'd have to have proof that he had taken them prior to testifying. And that would be difficult, short of videotape of him in a Rangers uniform shooting up.
Or if the test was taken earlier in the year.