This depends on the kind of cassette tape that we’re talking about — remember, there are video cassette tapes as well. That being said, if we’re talking about audio cassette tapes — the magnetic tape that this format uses contains an emulsion, whose density on your tape is the main factor in the audio quality and its longevity. As long as you listen to the tapes, again and again, you will wear this material out. Now, the problems with the tape will be readily apparent — the only question will be whether the tape itself is worn out, or if there is a mechanical issue with the tape and the playback machine. In the case of a mechanical failure, the tape will be too loose on the machine’s spindles; it will either fail to rewind or it will produce a plastic crackling when played. On the other hand, if the tape is worn out — you will hear missing parts of the audio, dropouts, and distortion.