Whom to contact
Referees should have had adequate opportunity to observe the candidate performing work related tasks. The candidate’s most recent immediate supervisor is a good choice, although they may not be available if the candidate is still employed and wants to maintain confidentiality.
A word of caution-be mindful of the source of the information provided and the self-interests of those involved. Some references can be inaccurate due to factors such as limited knowledge on the part of the referee, personal bias, self-interest in either retaining or in some cases wanting the person to leave the company but for all the wrong reasons.