Final answer:
The criteria for considering someone under the influence at an APO event include behavioral signs and physical symptoms suggesting impaired cognitive and coordination abilities due to substance consumption.
Step-by-step explanation:
The criteria for considering a brother 'under the influence' at an APO event involves observations of their behavior and physical symptoms that suggest they have consumed alcohol or other substances to a degree that impairs their cognitive functions and physical coordination. Signs include slurred speech, instability, erratic behavior, impaired judgment, and the smell of alcohol. It is important for fraternity members to monitor such behaviors to ensure the safety of all attendees and to adhere to the organization's policies regarding substance use at events.