There is no cemical difference between one form of methane and another. It's a real simple molecule with no room for fiddling the geometry or composition

H
|
H--C--H
|
H

Now, Isotopes are a different matter. (I'm working on a CO2 project right now where knowledge of isotopic ratios is required to identify the source(s) of the gas)

The Carbon isotopes of Methane on Earth are commonly used to diagnose biogenic methane ("light") from volcanic methane ("heavy"). If you can find coexisting gasses such as CO2 and CH4, the isotope ratios are very telling. (There are established ratios expected in equilibrium chemical reactions between these chemical species. Life is NOT an equilibrium reaction and fiddles with the isotopes,having a preference for lighter C12 vs heavier C13)

[On Earth, C14 is produced by cosmic ray impacts onto N14 in the atmosphere. C14 is the basis for Carbon dating of archaeological artefacts due to its convenient half-life, but since Mars' atnosphere is almost devoid of N, we don't expect Carbon dating to be of much value on Mars!)

One of the uncertainties is what the "normal" isotopic composition should be, but on Mars atmospheric CO2 should be good as a reference.

Further reading on carbon isotopes:-

http://www2.glos.ac.uk/gdn/origins/life/carbon.htm
http://www.gaschem.com/determ.html
http://www.gaschem.com/landfill.html
http://www.terranovatech.net/res_gasfund.shtml

More detailed:-

http://www-odp.tamu.edu/publications/164_SR/VOLUME/CHAPTERS/SR164_07.PDF
http://ijolite.geology.uiuc.edu/04FallClass/geo562/12Carbon.pdf