Identify the abilities of other gifted vampires
"He has the ability to sense the type and strength of gifted vampires' talents."
- ―Entry on Eleazar's gift
Ability identification is the Eleazar's power to determine if a human or vampire has a special gift and identify that person's ability.
Eleazar instinctively feels the gifts of vampires and humans alike. He considered it a "haphazard practice", as no one's gift worked in the exact same way because "everyone's mind is different". He is capable of getting a general idea of what any vampire is capable of just by being in proximity with him or her.
Eleazar was invited to join the Volturi coven for this reason ― Aro considered very useful the fact he could, potentially, determine which vampires had the strongest offensive gifts, and, therefore, determine the Volturi's most dangerous foes. Aro would often send him searching for rare talents like Bella's or Benjamin's to bring back to him therefore strengthening the Volturi. When Eleazar left (due to finding his mate, Carmen), Aro was very disappointed, as he was losing possibly the best defensive weapon the Volturi had. He only allowed Eleazar to leave in order to preserve his gift for later use because he believed that Eleazar would return to the Volturi if he was needed. Eleazar informed the Cullens of many Volturi vampires' powers during the confrontation with them.
Eleazar was the one who helped to discover Bella's own gift once she became a vampire, which was a mental shield. Unlike previous cases, Eleazar's gift was blocked by her shield; and this convinced him that she was indeed using some kind of powerful mental shield. This probably also means that he can sense when his gift is being blocked.
Eleazar can also sense the latent gifts of humans, though to an extent―he has to concentrate harder on a human, because the latent ability is so nebulous. And like Edward, Eleazar has to be in close proximity with his target to get a feel of their gifts.
Like the majority of vampire gifts, Eleazar's gift is mental; so his gift can be blocked by Bella's ability to block mental abilities. However, unlike most cases, he seems to be able to feel his gift being blocked.
- Joseph has the power to sense anything he wants, this includes being able to sense the strength of others