Certainly if we found an organism using a different set of bases, or encoding amino acids with different codons, itd be proof of aliens.
Its worth thinking about how DNA sequencing works, though. Any base that base paired with C would be read as a G, even if it was chemically different. Likewise any base that base paired with A would be read as T, etc. So its entirely possible the standard methods for DNA analysis would miss this detail at first, although Mass Spec would probably show it pretty clearly.
Don't get me wrong; the Mexican things are obviously fake, its just interesting thinking about alien DNA.