Can a spinal tap detect seretonin levels or any other drugs or alcohol?

Spinal Tap: Examining cerebrospinal fluid is usually to detect infection or inflammation, among many other diagnostic tests that measure different constituents in the fluid. Drug testing is not one of them, to my knowledge.

Not routine tests: Unless a doctor had a very specific reason to want to try to look at serotonin levels, this test is not offered at most hospitals. It may be only available on a research basis. The same can be said of testing for drugs and alcohol. Blood or urine tests are use clinically. CSF studies would most likely involved as part of medical research, if at all.