Answer
There are three ways of identifying a Mujtahid, and the A'alam (most learned):
*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 or the A'alam, confirm that a person is a Mujtahid or an A'lam, provided that two other learned and just persons do not contradict them. In fact, being a Mujtahid or an A'lam 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 or an A'lam, certify that a particular person is a Mujtahid or an A'lam, provided that one is satisfied by their statement.