John is probably right about the gaming, but let's play along here. What kind of evidence could support Froome’s claim? There are tests for kidney function, but not for after the fact. His claim seems to be he had acute kidney malfunction, but that the kidneys returned to normal by stage 18. So tests for kidney function on him now are unlikely to help.
So what could help? His urine samples prior to stage 18 now become extremely important. If his kidneys were not excreting substances properly—let’s say his kidney malfunction occurred during part or maybe all of the Vuelta—then his salbutamol levels for these earlier stages should have been really low, probably < 100 or even 50 ng/ml. If they were much lower than his levels after stage 18, and perhaps also lower than at other times in his career, say, at the TDF, that would be suggestive. But how good these levels would be as evidence would depend on how much salbutamol Froome was taking then, and of course he can’t prove how much he was taking. Unless there was a really sharp drop-off—say, from 300-600 ng/ml to < 100 ng/ml—this evidence would not be compelling. And there’s no reason to think his kidney malfunction would occur suddenly, unless it was triggered by some event, such as acute dehydration. I guess that’s possible.
So they would probably also look at other substances in the urine, such as creatinine. But this is complicated, too, because during the intense exercise of a GT, his creatinine blood levels will be much higher than at rest, and this will affect urine levels even if the kidney is not fully efficient. I would think even in a rider with normal functioning kidneys there would be huge variations in creatinine in the urine during a GT, and these variations will tend to obscure any consistent effect that kidney malfunction might have. They might want to compare creatinine levels in his Vuelta samples with those in earlier samples, e.g., from the TDF, to try to establish some kind of baseline.
But this is not the way one normally looks for kidney problems. One would want to measure serum creatinine levels, and urinary levels I think are only used if urine can be collected over a set period of time, not just in one sample given at the end of a stage. So Froome will not only be trying to prove that he had kidney problems after-the-fact, but with limited samples available. He might have given a blood sample during the Vuelta, which could be helpful, but one sample in isolation would be of limited value.
the night after stage 17, who then conducted despicable scientific experiments on him whilst hypnotised with Mozart playing gently, soft pillows and beetroot juice, and after inseriting their new alien DNA and organs they popped him back into the hotel bed sans original crazy adaptive alien kidneys just in time for a breakfast of champions 
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