How do we determine whether a dish is clean?

Question

There are situations in which the question arises in halacha (specifically, the laws of kashrus) whether a piece of flatware or a dish is clean — by which I mean whether it lacks any remnant of food or the like.

(Such a situation might be as follows. One can (citation needed, and CYLOR) use a cold, clean, non-kosher cup for a cold drink. But he cannot (c.n., a.C.) use a cold cup that contains non-kosher food remnants (because אין מבטלין אסור לכתחלה).)

My question is how one determines whether there are any remnants in the dish or on the cutlery. For example, is it sufficient to glance at the dish? Does one need to look at each square inch of it carefully? Perhaps run a fingernail along it to feel for adhesions? Or what?

Answer 1

There are two places in halacha where this question is commonly addressed: the laws of insects/forbidden organisms in food and the laws of cleaning for Pessah. In both cases the key criteria is visual inspection. The halacha is not concerned with what the eye cannot see.

For instance

On cleaning for Pessah

If one is not using the oven on Pesach: The aforementioned process is not required, one should visually inspect that there is no chametz that is fitting for a dog’s consumption, and close the door with tape.

On bacteria in yoghurt

The Torah prohibits only bugs which can be seen by the naked eye. Organisms seen only with the aid of a microscope are kosher.

(cites the Aruch Hashulchan, Yoreh De'ah 84:6 but I haven't checked it)

On bugs in vegetables

We are forbidden to eat bugs that are big enough to be seen by the naked eye.

as well as from the OU guide to bugs in food

[...] consuming a minuscule insect would not be considered an aveirah unless the creature is identifiable to the naked eye.

finally on worm in fish by Rav Belsky

any organism not visually discernible by the eye has no meaning in halacha

Also see here for a very interesting article on the halachic impact of the microscope being ... microscoping in light of what is written above. He adds that etrogim and sifrei Torah also do not require more than visual inspection.

But the bottom line is that using a magnifier or microscope to see something that cannot be seen by the naked eye would have no halachic bearing “bein lehakel bein lehachmir”

So from all these sources it looks like cleaning followed by visual inspection is the key criteria used by halacha. As always CYLOR for practical cases.