Carbon 12 isotopic evidence
Carbon has three isotopes – Carbon 12, Carbon 13 and Carbon 14. The second is the most abundant. Despite that, many organisms accumulate Carbon 12. All life we know about is made from carbon. The increased concentration of Carbon 12 in a given sample can be used as a signal of life, but it is not foolproof.
When organisms die, some get buried in the sediment and then the sediment turns to rock in the Earth's crust. Some of the carbon is preserved and its isotopes can be measured. Right back to 3.5 billion years and perhaps even older the abundance of carbon isotopes suggests the former presence of life.