What analytical signature acts as a universal litmus test for biological activity regardless of local conditions?

Isotopic ratio of heavy to light isotopes (like C-12 vs C-13)

While finding organic molecules like amino acids is suggestive of prebiotic chemistry, determining if actual metabolism occurred requires a more definitive chemical marker. The ratio between heavy and light isotopes, such as carbon-12 versus carbon-13, provides this universal analytical signature. Biological processes exhibit a phenomenon called isotopic fractionation, where living organisms preferentially use lighter isotopes, leading to a distinct ratio that differs markedly from ratios produced by purely abiotic (geological or radiation-driven) processes. A strong biological fractionation signature offers much more compelling evidence for past or present metabolism than the mere existence of organic compounds.