There are laboratories measuring that stuff from urine and saliva. They are expensive and the results also need to be controlled for things like normal thyroid function, so you can map properly derive levels in the brain. Also the tests need to be done at different times of the day.

The test-results include ranges. Interpreting and correlating the values and if they are problematic is the work of the few doctors who work with these labs. A bit like with cholesterol levels in the blood.

A doctor once told me that you can't tell for sure, and there are complete surprises, but normally, if she sees a result-sheet she can give a relatively accurate guess about how the person "generally feels" over the day (tired, happy, hungry, anxious, stressed, relaxed, trouble sleeping or not etc.)