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. 
No. There are so many products used in so many different ways in crop protection and as preservatives for harvested fruit, that it is impossible to say beforehand what the most likely contaminants are and how to test for them. I was hoping only waxed fruit is treated with preservatives, and that i could test for wax easily, but it is impossible to say if a lemon was waxed, and unwaxed lemons may be contaminated also. I have not even been able to find a top 10 list of most-often found pesticides and preservatives on/in lemons. The name that does come up very often as a preservative applied after the harvest is Imazalil. The up-side is, it can be washed off to some extent. But this only focuses on the zest, and unfortunately 1/5 of Spanish lemons (and 1/2 of the Turkish) contain chlorpyrifos, which is neurtoxic pesticide that will be banned at the end of this year (2020). It is added to the ground so it is in the fruit, not just on it. In general however most of the preservatives and pesticides stay within safe margins (Dutch link). If the farm also sells to supermarkets and such the product will have to be clean enough to pass tests, which most fruit does. I have found little info about what organic farmers use to protect their crops, what i found seems pretty ok (Dutch link). Also the lemons are in season currently so they are anyway less likely to be treated after harvest. The best options are to contact the farm and decide how much to trust them or to rely on store-bought food that has some labelling and certification. 