You test it for various common and uncommon cases. Like how does it influence fruit fly fertility if the concentration is above x%. But nothing is 100% sure in

You test it for various common and uncommon cases. Like how does it influence fruit fly fertility if the concentration is above x%. But nothing is 100% sure in the real world, if that was your point, but for practical matters we can get close enough, if we want to. But I know that quite some tests are not being done, because the results would not be welcome.