One of the more common (and potentially successful) suicide attempts is to drink a bunch of antifreeze, which contains ethylene glycol. As we all know, this is typically diagnosed by characteristic lab abnormalities: patients present initially with an osmolar gap, and as the ethylene glycol is gradually metabolized to the organic acid oxalate, it evolves into an anion gap metabolic acidosis.
Another trick for diagnosing ethylene glycol is to use a Wood's lamp--which emits ultraviolet light. Most commercial antifreeze contains a compound which fluoresces under ultraviolet light; this is included so that car mechanics can detect potential antifreeze leaks. Thus, it is theoretically possible to detect a recent ingestion of antifreeze by seeing whether or not a patient suspected of an overdose is fluorescent under uv light. I use the word "theoretical" because there is some literature out there such as this which cast the sensitivity and specificity of uv-fluorescent urine into doubt.