Unfortunately there is not full proof test--short of autopsy! Many people with only suspicious hair tests plus a half a dozen mercury symptoms do well with chelation. You are looking for clues from various tests, symptoms and reaction to taking a chelator. People who have an Hg problem usually have worsening of symptoms while on round.
A close friend is a good example. she feels more tired and fogged while on round but also sleeps better. Her pre chelation hair test is not so bad--#368 at the site above. By that time she had found a decent combo of medicine, supplements, lifestyle and diet that had improved her symptoms somewhat. Cutler indicates it can make a hair test look better as well.
Notice her first hair test in the series from 10 yrs prior to Amalgam removal . It is a text book case of mercury! This was from the time when her symptoms were at their worst and she had not yet found a nice combo of support. She is gradually getting better, although not without some bumps in the road.
Also my prechelation hair test--#354--was not too bad. However followup tests a 1 and 1.5 yrs reported either red high Pb or Cd. We often see hair test look worse after 6 months of chelation or so as the heavy metals get kicked up. #360 is a good example. After his second hair test he was convinced that he had a mercury problem. The second test matched one or two of cutler's rules. More on an early version of them here. http://onibasu.com/archives/am/2200.html
If $ is tight. You could try a round of chelation, maybe 10mg DMPS if you can get it or 5-10 mg DMSA and see if you get side effects. DMSA is generally not good when candida is a problem. More on that if requested and in the Files of the Frequent Dose Chelation yahoo group. Also Check out TK's General Suggestions for dosing suggestions if you are going to use Cutler's protocol. I certainly recommend it after my nice gains. //www.curezone.org/forums/fm.asp?i=1951581
Good luck. Just don't delay unnecessarily. I wish I had your opportunity when I was in my late 20's. I wasted my 30's chasing Lyme etc and then giving up before my friend got me pointed in the right direction around age 40.