Methane (CH4) is a compound with molecules each consisting of one carbon and four hydrogens atoms. It is produced by biological processes on Earth, and is generally unstable, thus constitutes a biosignature. It can also be generated abiotically, but its observed presence suggests some long-lasting process is generating it. It is observed on some solar system moons.
If the molecule contains a deuterium atom, which can be called CH3D, then methods of tracing isotopes (e.g., varying spectra, or varying mass) can be used to characterize observed chemical reactions. This has been done using spectrograms of Titan's atmosphere taken by Keck Observatory.