They do not have to make the proof public, in fact because of privacy issues they are NOT supposed to.
BUT Jeremy and his lawyer know and have the toxicology reports that "IF" he is truly innocent they could prove to the world with them.
NASCAR can not tell what it was because it infringes on Jeremy's privacy. For the same reasons Drs. and hospitals can not release to the public your medical condition without your permission, but you can tell the world if you want to, and so could Jeremy.
Aren't you wondering why this hot slick attorney hasn't filed papers with the court? "IF" they were holding the proof Jeremy was innocent don't you think they would have immediately filed an injunction to force NASCAR to let Jeremy race until they could get their court date?
If it walks like a duck, quacks like a duck, smells like a duck, it is usually a duck.