I'm in a similar situation here looking for a lab which will test for specific chemicals. You'll need to contact soil labs in your region and ask if they will test for the specific chemicals you're looking for or if not, if they can refer you to a lab who does.
You could call a university with a good ag programme and ask for a good lab in your area.
Around here, the University of Guelph has a good ag programme and a lab, but they were able to run the lab commercially while also getting government funding, which made it easy for them to under-price the commercial labs, so I'm not a big fan. They do good work, though.
A piece of land is worth as much as the person farming it.
-Le Livre du Colon, 1902
I am also wondering how one would decide what to test for when little information about potential contaminants is available.
In my case I am not worried about herbicides/pesticides; the state of the property, history recounted by neighbours, and vigorous growth of broadleaf plants in even the most respectable of the 'fields' all argue convincingly that no deliberate application of anything has happened for at least one and probably 2-3 decades.
I do wonder about contamination from a long ago dump area, obviously at some point garbage was piled up from the original homestead, and later a mechanized farm and small sawmill.
How worried should I be about burn pile residue mixed with my soil, in food-growing areas.... when I have no data about what/when was burned?
There is an area that is suddenly dead of nettles, weeds, and grass that was vibrant and alive two weeks before. The line where this death is very marked and rectangular.
I thought at first a bit of wood was put there, but there's none of the leggy white growth that happens when something like nettles grows underwood.
Not sure how to test for this but I can't think of anything non-chemical that would cause this damage.
I would ask for tests for petroleum products and heavy metals. Those are the most likely contaminants. You can do your own tests for herbicides by growing seedlings of plants which are especially susceptible to them such as beans and tomatoes. I did a test for aminopyralid, a persistent herbicide, using a tomato seedling: https://permies.com/t/113586/Herbicides-hay
The persons involved may be pursuing criminal or civil charges. No lawers yet, but it's a history of behaviour that led up to this and the individuals I'm advocating for want documentation that is acceptable to the police or the judge if things escalate.
Does this change the documentation and tests we need?
What sort of plants are present, and were all impacted? This might at least rule out some sorts of herbicide that only work on some types of plant..
I would expect a test result sheet from a lab to carry a lot more weight than the standard DIY herbicide test, should this end up in front of some sort of Official Person. Official People seem to really like Official Looking Paperwork. It seems likely to also provide more specific information.
It might be possible to do the DIY test to confirm that it seems to act like an herbicide and then send off for a lab asking only for herbicide testing to save a few bucks?
On the documentation front... more is better! Lists, dates, corroborating witnesses, actions taken, photos, texts or voicemails received... keep everything, and move forward with an eye out for the opportunity to collect more.
These are not the droids you are looking for. Perhaps I can interest you in a tiny ad?