One of the simplest ways to detect cold interstellar gas clouds is by taking spectra of distant stars. These spectra will show the absorption lines indicative of the spectral type of the star, but may also contain additional absorption lines if the light has passed through gas along the way to our detectors.
These interstellar absorption lines are created when cold interstellar gas absorbs some of the radiation emitted by the distant star. They tend to be much sharper and more narrow than the absorption lines created in the atmosphere of the star, and indicate the temperature, density and chemical composition of the interstellar gas through which the light has passed.