How can you tell when a guitar string is old?

Answer 1

Old guitar strings sound dull - they lack the tonal richness of fresh strings, whether they are nylon or steel.

Older strings are also difficult to keep in tune; their lack of elasticity will mean that older strings will often go audibly off-tune even after a few minutes continuous playing.

In the case of metal strings the strings will also look dull, and will be prone to metal-fatigue (making them more likely to snap during tuning or playing).

---------------------------

Assuming were talking about steel strings, some musicians replace them after each performance or practice session. There is no accurate answer. As a rule, a casual player should replace the strings at least once a year and some would say 6 months. If you look closely or rub them and there is any corrosion at all - they definitely need to be replaced because their ability to produce accurate tones is compromised. If someone gave you and old guitar and you think it sounds pretty good, play it until one of the strings breaks.