Perhaps aliens eat right handed life, but left handed life is poison to them.

Seriously. It would be a pretty good selector, and said “alien” need be no more than a snippet of RNA - and it would be entirely gone from earth now, eliminated by us sinister life forms.

The only evidence would be the ubiquitous absence of R-entantiomers in life.

I think I might be lifting from Asimov - The Left Hand of the Electron.

That kinda kicks the can down the road though, because we are faced with almost the same set of questions except about the hypothetical alien life.