As far as i know, an NDA is just a contract. Breaking it is no more illegal than defaulting on your credit (since it seems to be a common topic): you break the contract, pay the fine or whatever. Disclosing trade secrets is something else however
I'm pretty sure that if you see something illegal, you must go through the NDA and go to the police
Other than that, just read the contract, or show it to a lawyer. Finding lame technicalities to invalidate an act is their job