The threshold
Any material proof is insufficient. Any material artifact, construct or device would be insufficient evidence of an inhuman intelligence. The only thing an advanced "alien" can bring to Earth is knowledge and for the most part, such would go unnoticed in much the same manner as x-rays. So "extraterrestrial" intelligence will go completely unnoticed by man.
Man has to reach out and find our neighbors, and when man can do that there will at least be some parity in development. Until then any landing, interaction or contact would be like placing a Saturn stage in a child's playpen - less than useless and merely destructive. Worship, fear and utter confusion would result.