The cloudy head (brain-fog) was a big problem from me after receiving 7 fillings. It's a common complaint for every mercury toxic person I have ever spoke to. The rest seems to vary - but alot of things crop up like

* poor immune system (always getting ill)
* brain fog
* gluten intolerance
* feeling more tired than normal. never really feeling refreshed after a good night's sleep

If most of the above tick the box, it could be the mercury. Forget a challenge test, hair analysis is probably the safest. But to be honest - it's going to be a gut instinct thing at the end of the day if you've eliminated all other possibilities.