How Google can come clean on PRISM. Suggestion.

If Google has nothing to hide, there are ways to show it.

Allow few selected organizations like EFF to do security audits on Google. Auditors, who are domain experts, sign NDA just like any consultant would do. Only the result of the audit and conclusions are public. Trade secrets stay safe. If they have nothing to hide, they can even afford to pay for periodic audits it and keep their name clean.

Massive bulk surveillance like PRISM will show if the audit is comprehensive. It needs infrastructure and bandwidth. There might be people sending anonymous tips on where to look.