The visible and smell-based signs
The most reliable signs are the ones you can see and smell. Watch a register the moment the system kicks on — a visible puff of dust means there's a reservoir inside. A musty or stale smell when the AC runs usually points to biological growth on the coil or in the ducts. Visible mold anywhere near a supply or return vent is an immediate reason to inspect.
The performance and health signs
Some signs are subtler. If surfaces collect a film of dust again within days of cleaning, your system may be the source. Unexplained allergy or asthma flare-ups that ease when you leave the house can point to indoor air quality. And rooms that never seem to get enough airflow can indicate restriction from accumulated debris.
When in doubt, inspect first
You don't have to guess. A reputable contractor will photograph the inside of your ducts, air handler, coil, and blower before quoting anything — so you can see the actual condition rather than take a salesperson's word for it. If the system is clean, an honest company tells you so.