It's obvious when a ribbon is "blown", the mic will suddenly lack highs and lows, and its level will drop substantially (6 to 8 dB). You may also hear mechanical rattling sounds, which comes from the ribbon element scraping against other parts in the ribbon transducer. There's usually no doubt when a ribbon is blown - you'll know it when you hear it. A completely blown ribbon (ribbon element torn apart - very rare) would have no output. Fortunately, replacing the ribbon element will bring the mic back to new condition. Ribbons are designed to stretch over time with no negative effect. A little ribbon fatigue can actually sweeten a ribbon mic slightly. With normal use, ribbon elements will last many years before needing replacement. The mic will perform to spec until the ribbon is overly stretched, at which time its performance will fall off rapidly and you'll know it's time for a re-ribbon. The #1 way to blow a ribbon mic is to loan it out! Engineers who own ribbon mics and read their manuals know how to handle them and rarely damage them. The person you loan your ribbon mic to probably doesn't have your level of ribbon expertise, or possibly won't be as careful with it because it doesn't belong to them. Watch who you loan your ribbon mics to, or which engineers in your studio you allow to handle them. Other common ways to blow ribbon mics are dropping them, exposing them to wind (using them outside without a wind filter, placing them near vents or doors, etc.), and exposing them to phantom power (see Can phantom power damage a ribbon mic?).