It often goes unnoticed by police because they concentrate on underage possession. But a plain-clothed officer who observes a person buying multiple drinks can easily watch where the buyer brings the drinks and then check IDs when possession takes place. It's often up to the officer to determine who at a table is in possession of alcohol. The criteria used by officers is whether a reasonable person would conclude that a drink belongs to a person when making arrests. So even if a minor pushes a glass away, it's up to the officer to say whether that person is in possession.
Officers often watch bar employees for illegal activity as well.
Even if the minor does not heed the advice, the employee is held responsible. It was not uncommon years ago for bars to announce the arrival of police so minors could get rid of their drinks before getting caught. Both cities now have laws against such announcements, though Friedlein said he has heard that some employees use code words or phrases to announce to customers the arrival of the police, though the practice is not as common as it used to be. A class K permit is required in Champaign for anyone who purchases more than one 16-gallon keg of beer for a party or private use. The license is obtained through the city.