One should test, of course. But few do.
At a minimum: the neurotransmitters, yes, but where? Dopamine in the gut - does it relate to the brain? what about ratios rather than absolute values? Does the level of "x" really matter so long as there is three times as much "y"? So, don't just run into the lab and throw money around. Think it through and have a reason for your decisions.

Don't stop with the brain neurotransmitters, though. In fact, don't even start with them unless you have the reasons. There are other tests that may be of more value to you. For example, There is ample reason to think that PD is driven by inflammation in the CNS. C-reactive protein gives info on the peripheral conditions, but what is going on past the blood brain barrier? Maybe you could learn more by tracking TNF-alpha or another cytokine. Keep in mind that these chemicals are neurotransmitters in their own right, too. We have had the opportunity to observe one of our number go down fast with an abscessed tooth from those chemicals and to bounce back as the antibiotics went to work.

Then there are the similar chemicals of the endocrine system's stress circuits. Cortisol is a reflection of chronic stress, but it is also a reflection of inflammation. Adrenaline is a marker of acute stress, but it can play hell with electrolytes, too. And don't forget the electrolytes! They are responsible for the functioning of both nerves and muscles. Yet, no one tests for them.

Look online and figure out how wide you can cast your net. Maybe you can find a sympathetic GP to give you a break on costs for drawing the specimens.

But share your data here. -Rick