WordNet sense 1 (someone who gives a warning so that a mistake can be avoided):

reminder, admonisher

WordNet sense 2 (someone who supervises (an examination)):

proctor

WordNet sense 3 (display produced by a device that takes signals and displays them on a television screen or a computer monitor):

monitoring device

WordNet sense 4 (keep tabs on; keep an eye on; keep under surveillance):

supervise, ride herd on

WordNet sense 5 (any of various large tropical carnivorous lizards of Africa and Asia and Australia; fabled to warn of crocodiles):

varan, monitor lizard