I am of the same opinion as Manjushri (the person who started this post) and I think there is no way to know for sure unless we are enlightened ourselves!
One can refer to their biography for reference, or what I have always used as examples are what great Lamas said, but again for people who don't believe, they can choose to dismiss that as well.
The standard biography of the Second Dalai Lama (started by Yangpa Chojey in 1530) begins with the legend of Avalokiteshvara, the Buddha of Compassion, of whom the Dalai Lamas are said to be emanations. He mentioned how in ancient times Avalokiteshvara vowed to take care of Tibet; how he (Avalokiteshvara) incarnated as various kings in order to bring the enlightenment teachings to the country; and also “in the middle he came as Lama Drop to complete the work of Jowo Atisha; and finally as Gyalwa Gendun Drup (the First Dalai Lama0 he made firm the doctrines of Lama Tsongkhapa”.
Atisha once said in reply to his disciple, Ngok Lotsawa:
“Well, Ngok, as you know, Lama Drom is an incarnation of Avalokiteshvara, the Buddha of Compassion, so make a symbolic offering of the universe to him…”
Atisha then relates to Ngok Lotsawa the 36 “previous lives of Lama Drom”, each beginning with “…as you know, Lama Drom is an incarnation of Avalokiteshvara”. These 36 are embodied in chapter twenty-three of the Kadampa classic Father Dharmas Son Dharmas (Tib., Pha-chos-bu-chos).
During his lifetime the First Dalai Lama was regarded as being the reincarnation of Lama Drom. Thus the connection of His Holiness being an emanation of Avalokiteshvara was linked to the literature mentioned above as spoken by Atisha.
You can also relate what was spoken by Dagpo Rinpoche about Trijang Rinpoche and His Holiness (as Avalokiteshvara) working together to bring Buddhadharma to many in this thread on this forum:
Read what Dagpo Rinpoche says about Trijang Dorje Chang,
http://www.dorjeshugden.com/forum/index.php?topic=1985.0Personally, the easiest way to judge whether a person is enlightened or not is to look at the results of activities. If their being somewhere and doing something has brought tremendous benefits and Buddhadharma to many, then their 'job' is the same as the enlightened ones.