Chris Haynes
Perfectly Poisonous Person
If you go through www.pubmed.gov you will see several cases studies of actual mercury poisoning. Some of them you can access the full paper (the others will require you to go a university library to get access through that library's subscription).
Here is one: http://www.cmaj.ca/cgi/content/full/168/2/201
That one uses a blood test... which would show the levels in the blood, which if the chelation was by IV, that is where the chelator would seek out the mercury atoms.
As a bystander I think the best way to go about a study of this would be to do a couple of different tests. Take samples from hair (which would show exposure over time, and is really not the best stuff), urine and blood... BEFORE anything is done. Get all levels.
Then take periodic readings during the period of treatment.
Have the analysis done at a couple of places to make sure of quality. Like two different hospital labs. Also, do not rely on the parents to provide samples (like hair samples).
Making sure to take in the height, weight and age of the child. Also, at the same time a short series of motor function, speech function and simple evaluation tests would be done on the child. From (depending on the age) tracing a drawing to matching shapes.
By the way, I am basing this on my past life as a structural dynamicist, and how we conducted preliminary parameter studies (change one variable at a time, see what happens). subsequent tests (measure as much as you can to get a full picture of what is going on) and the followup analysis with the garnered test data.
Also, for your amusement and edification (I learned from it) check on the video explanation of the Burbacher, et al paper:
http://bartholomewcubbins.blogspot.com/2006/01/bartholomew-cubbins-on-autism-episode.html