There are several signs that indicate an air conditioning (AC) filter needs to be changed. Reduced air flow from the vents, higher energy bills, and the presence of dust or debris around the vents are all indications that the filter is dirty and needs to be replaced. Additionally, a musty or stale odor coming from the vents may be a sign that the filter is clogged and not allowing air to circulate properly. It's recommended to check and replace AC filters regularly to ensure proper operation and energy efficiency.