Know if lemons are safe to eat whole (peel and all)

Is there any way to tell if lemons contain a residue of something that is harmful to humans?

The last two winters we have been buying oranges in bulk through a website, directly from the farm and i really like it. I would like to buy lemons in bulk too.
They say their lemons are not treated with anything but I would like to have a little more reassurance. The fruit is not certified organic.

When buying in a store I assume the person buying the fruit for the store tests occasionally and consumer organizations and governmental organizations also sometimes test.
But now I will be buying directly from the farm, without the middleman so to speak.

Related:
Lemons and oranges coated with imazalil

EDIT:
The type of answer I am looking for is how can I test, at home, in the kitchen if the fruit is covered with something like fungicides. I know citrus is often waxed, but that can be washed off, and I will wash the fruit anyway before eating.

Alternatively, maybe pesticides and fungicides are so expensive that farmers are likely to avoid it if they can. Remember, these fruits are shipped straight from the farm, only fruit that is in season is sold, so there is no reason to keep them long.