Conclusive evidence of intelligent extraterrestrial life will be detected by 2050.
Terrestrial-origin entities (e.g. colonists, biological constructs, computational constructs) whose predecessors left earth after 1900 do not satisfy this claim. If the intelligence of the ET is not obvious, the primary judging criteria will be either a significant level of technological sophistication (e.g. radio transmitting capability) or conceptual abstraction (e.g. basic mathematical ability). Radio signals received or similar tell-tale signs of intelligence (e.g. archeological discoveries) detected and accepted by scientific consensus as originating from intelligent extraterrestrials would satisfy the claim even if not completely understood by the claim judging date.
If the manifestation isn't obvious (e.g. ID4-type contact) I intend to rely on refereed scientific journals (e.g. Science, Nature) or other generally respected scientific sources for a reading on scientific consensus.