There is a Universe Today article on the lack of organics found by the Voyager missions, which suggests that the chlorinated organics originally thought to be cleaning products could have actually been reaction products from Martian organics and Martian perchlorates (perchlorates and organic molecules react vigorously when given the chance- see the Space Shuttle’s solid rocket boosters for an example). The paper is not yet available, and I’m not a big fan of the press-release-before-publication model that NASA seems to be getting fond of these days. So I’m gonna sandbag the study based on the interview.
“One reason the chlorinated organics found by Viking were interpreted as contaminants from Earth was that the ratio of two isotopes of chlorine in them matched the three-to-one ratio for those isotopes on Earth. The ratio for them on Mars has not been clearly determined yet. If it is found to be much different than Earth's, that would support the 1970s interpretation.”
So, measure the martian Cl isotopic ratio and see in the Viking measurement is terrestrial or martian.
This is not a well designed experiment. We know from Sharp et al. (2007) that Cl isotope ratios don’t vary much either on Earth or in chondrites. One permil is 1 tenth of a percent, so a 1 permil difference is a change in 35Cl/37Cl ratio from 3.125 to 3.128. It is unlikely that the Voyager measurements are anywhere close to precise enough to see this sort of variation.
Long et al. (1993) show that surface processes on Earth change the ratio by up to 4 permil. Nakamura et al. (2009) show a similar change in Cl metabolized by organochlorine-eating bacteria. The only place Cl isotopic variations exceed 1% (ten permil) is the moon (Sharp et al. 2010), and this is due to an extremely hydrogen-poor environment (insufficient H for chlorine to volatilize as HCl when outgassing from magma) that we know doesn’t exist on Mars, as it is covered in ice and hydrous minerals.
Luckily, we have Martian meteorites. They have lots of Cl in them (Bridges et al. 2001), so we should be able to get Cl isotopic ratios for phases present in those samples. I suspect that Sharp might be doing that as we speak. But there is no reason to expect that the values will be different enough to terrestrial that they can be used to test the provenance of the Viking Cl isotopic analyses.