You don't say which chemicals you are interested in!
A Google search using 'brain chemistry' as your search term will yield many web sites which contain a lot of information about normal brain chemicals called neurotransmitters, and how imbalances in them can cause depression or mania or bipolar symptoms.
As far as I have been able to discover there are no techniques to determine amounts of each of the neurotransmitters a person may have at any time.
Rather, it is by the use of some simple questions that a determination of the overall state of a persons mental health is found. Relevant antidepressants or other psychiatrical treatments are then used in an attempt to bring the neurotransmitters back into balance (whatever has caused the imbalance).