If you are talking about live, active soil samples, then production of carbon dioxide and loss of oxygen via cellular respiration is a sign of life. In particular, if you boil the sample (which should kill all organisms), and the carbon dioxide production disappears, you have disproven the hypothesis that CO2 is formed by abiotic processes. Now, if you are trying to discriminate between the presence or absence of compounds as a sign of prior biotic existence, there, I don’t have a great answer, but perhaps you can look for the presence of nucleic acids, amino acids or complex sugars. While these can be formed by abiotic processes, it is far more likely to be a sign of prior life.