Arsenic remains in the system after a person dies. Thus, a pathologist can check for it as part of an autopsy. If someone dies suspiciously, or if foul play is suspected, it is routine for the pathologist to test the body for several well-known poisons, including arsenic. The results of this testing are key evidence.
Arsenic poisoning can be detected through urine analysis, in the case of recently ingested poison, or by testing hair, nails or body tissue to determine exposure over a longer period.
Here is a short list of symptoms: drowsiness, headaches, confusion, changes in fingernail pigmentation, vomiting and diarrhea, hair loss, stomach pain, blood in the urine and convulsions. Hair analysis is the best source for detection.