Tricky question. This is a tricky question, as there is no way to measure serotonin in the brain synapses of living humans. Many (not all) antidepressants increase synaptic serotonin, so many people assume depression must be due to "low serotonin." there's no direct evidence for this. Some antidepressants don't act on serotonin, at least directly, & we literally don't know how any antidepressant affects mood.
Depression. People who lack seratonin are often depressed and/or anxious. Proper diagnosis can be obtained from a psychologist or psychiatrist evaluation.