Ask for original paperwork or service records if the monitors were recently repaired.
Play a track you know intimately to check for clean, uncolored sound at both low and moderate volumes. Logistics and Risks Buying "unseen" online carries higher risk.
Use a sine wave sweep from 20 Hz to 20,000 Hz to detect anomalies, buzzing, or voice coil rubbing.
Slide switches and turn knobs to ensure they feel firm and click into place without resistance or "scratchy" noise. Verify that RCA, XLR, or TRS ports are not loose or rusty. Functional Testing Protocols Testing should go beyond simply "hearing sound."
Inspect the woofer cones and tweeters for dents or tears. Check for bubbling or peeling finish, which may indicate exposure to moisture.