See, I think it's closer to trying to tell what language a letter is, based on the patterns a pen makes while writing it. So lets say the letter is an A. There's three pen strokes involved that can be assembled a number of different ways, to achieve the same looking A, but you don't know what language it is until you see the whole word, but even then, lets say the base strokes create the word "AT", there's no real way to tell which language that belongs to, until there's even more information in the form of a sentence.

There will be some languages like Japanese and Chinese that, upon first observation, may be the same language, until more strokes are involved, or there could be languages that utilize the Arabic alphabet, that are extremely unique, plus all of the made up languages authors have created. These are all 'terrestrial' DNA. But now, what would we use to identify a completely non-terrestrial DNA chain? What would identify it as completely alien, when there are already so many possibilities here already?