Officially it'll be "called" when a politician feels like it's solid enough proof for them to leech off of it. Scientifically, it'll be called when there's too

Officially it'll be "called" when a politician feels like it's solid enough proof for them to leech off of it. Scientifically, it'll be called when there's too much evidence to genuinely dispute it. Probably when a collection of specimens is found, either dead or alive, that can be shown to be unrelated or distantly related to Earth life.