If toxicity depends on highly individualized factors, then what basic evidence should a person look for to indicate that it could be affecting him?
Aside from clear cut cases where removal of a suspect substance is immediately associated with better health, one's evaluation of the impact of toxicity isn't so much about getting conclusive evidence, but about stacking the cards favorably in terms of the totality of your lifestyle.
As with many things related to health, the cumulative effect over decades of affecting something seemingly marginal, is often astounding. This is why you can have two fifty-something siblings (or twins), where one is a health wreck, and the other perfectly healthy and fit. (The health wreck is typically the person who only thought in binary terms about health: "My doctor says I'm healthy, so I don't need to do anything", whereas the fit sibling is the one somewhat obsessed about health optimization.)
People ask me: "You look so young for your age, and you are so healthy, so why are you so picky with diet? Why do you take all these lab tests? Why do you eat fistfuls of supplements?". My answer is "Duh."
If farm workers who were exposed to pesticides got "alterations of the digestive, neurological, respiratory, circulatory, dermatological, renal, and reproductive system", and my goal is to be continue to be healthy and youthful for as long as possible, why shouldn't I want to minimize my own exposure to these substances if I can do so with minimal financial hardship or other negatives?
Anyway, I shall stop dodging your question. Here are some suggestions if you want some hard data to work with:
1) Check your genetics. Do a 23andMe. There are a couple of polymorphisms or SNPs such as APOE and MTHFR associated with poor detoxification capacity.
2) Run functional lab tests to assess your toxic load and excretion rates. Examples of such tests are hair minerals analysis, urinary organic acid tests, and tests for urinary lipid peroxides and urinary bile acids.
If your detoxification/excretion capacity is low, then you have a stronger case for working on reducing exposures and on improving detoxification/excretion.
Note that as a matter of sound therapeutic philosophy, removing overtly- or potentially harmful items (as determined by what our bodies could be reasonably exposed to be exposed to in evolutionary times) has the least inherent risk, whereas adding things like medications and supplements have higher risks because of their inverse U shape dose/benefit curves.
The first consideration in health is always to look at eliminating bad "stuff", before adding good "stuff" (over and beyond baseline nutrition).
Personally, since I have rather good genetics and decent hormonal- and gastrointestinal health, I don't actually obsess over buying pesticide free veggies, but rather take a middle of the road approach: I get some of my veggies form a CSA for part of the year, and I buy organic when a particular item is on the EWG's dirty dozen list. I supplement periodically with detoxification supporting supplements, and I do regular functional lab testing to monitor the status of important systems in my body.