The life doesn't need to be intelligent to count. Can be microbes on Mars or some exoplanet. I define strong evidence for this market to mean that a majority of scientists would agree that it's aliens and not something else (note: not consensus, just majority). The reason I'm setting this standard is that I don't think it will be realistic to find conclusive, incontroversial evidence.

Some examples for what counts as strong evidence:

• Biosignatures on an exoplanet, once the studies showing them have been replicated, confirmed, and accepted by the scientific community.

• Similar level of evidence inside the solar system. A confident announcement by NASA can also count, as long as it's not "maybe we found aliens" but rather "we are confident that we found aliens"

• UFO evidence that has been shown to have intelligent extraterristrial origin (just being unknown is not enough), well-studied and accepted by scientists.

What doesn't count:

• Some study showing what might be a biosignature on another planet

• NASA saying that they found something that might be associated with life on Mars or another planet.

• UFOs that are confirmed by the US government, but there's no significant positive evidence that it is, in fact, aliens

• Mexican aliens, unless there's some very significant new evidence released.