There are an awful lot of possible adulterants out there too.
Therefore, testing is a bit of a losing game. Instead, developed nations like the US require record keeping (ie. you need to keep a list of suppliers for each batch, and sometimes samples of each batch), and the court system allows someone to sue for a lot of money after the fact if a poisonous adulterant was found.
The risk of being sued is what keeps most food safe.
Is there a specific law that specifies damages? Because if not, it probably falls into the same civil framework as everything else where the amount of money sought has to be proven as proportional to damages. Unless it severely sickened you, it'd be difficult to quantifiably prove the damage that sawdust caused you.
And a good prosecution would be able to prove that while a small amount of lead didn't cause noticeable symptoms to anyone, there is plenty of evidence that across populations lead causes higher criminality, lower IQ, lower earnings, etc. And therefore, as a class, these people have been harmed by an actual dollar amount.