spectral signature for chlorofluorocarbons are obvious
I've read that CFCs are easy to read in atmospheres, so a Terrestrial Planet Finder type of telescope could detect them. In our atmosphere the time window for CFCs will likely be only 100 years or so, so the chance of seeing them on other planets is very small.
On the other hand, we might be able to detect spectra for other pollutants in exoplanet atmospheres that like CFCs have no known natural source.