Urban Land and recognizing toxicity

Hello,
I am part of an urban community garden that is attempting to be a permaculture site. We have been told by the city that the land could be contaminated with toxins but it would cost thousands of dollars to test for each of those potential hazards. So, being a community initiative, we are unable to pay for such testing. The soil itself feels amazing and rich. It has incredible weed species growing like dandelion, burdock, clover, mallow, yarrow, grass and more. Does anyone know how to identify whether soil is toxic simply by the plants growing there?
Thanks a lot!

I don't believe it will cost thousands of dollars for tests.  Costs seem to run $30 - 150 per test.  Lead is the most common urban toxin.  I think the city may be trying to discourage you from gardening.

http://grist.org/food/are-your-urban-veggies-really-toxic/

I agree about the tests. Also, I grow on an urban lot, tested for positive lead in two places. My solution is to plant trees and berries, my research indicates lead is taken up into leaves and roots but not the fruits and seeds.
For leaf and fruit crops, 18" of raised beds with new soil is plenty deep.
Lastly, I am adding wood chips and autumn leaves, as deep as possible.
It was am easy choice to simply avoid planting into my soil, as it contains the remains of a demolished house, and every shovelful of dirt is 75% urbanite rubble.
But you should test your soil in multiple places, and hopefully you can just plant into the ground as you please.

Lead especially is a cheap test, it is well worth it. Did the city way what it might have been contaminated with?