Hawzah News Agency - According to Ayatullah Sistani's risalah:
when a person is certain that a particular person is a Mujtahid, or the most learned one?
For this, he should be a learned person himself, and should possess the capacity to identify a Mujtahid or an A'alam;
when two persons, who are learned and just and possess the capacity to identify a Mujtahid, confirm that a person is a Mujtahid, provided that two other learned and just persons do not contradict them.
In fact, being a Mujtahid can also be established by a statement of only one trusted and reliable person; when a number of learned persons who possess the capacity to identify a Mujtahid, certify that a particular person is a Mujtahid or, provided that one is satisfied by their statement.
Source: https://www.sistani.org