Lisa-

You can measure with a ruler - on a new chain, it's exactly 12" from middle of pin to middle of pin. If it has stretched to more than 12 1/8", then you are looking at a chain and cogs likely (according to my LBS). Of course, you could choose to run it until the chain and cogs need to be replaced, and wait until the performance suffers (that's another school of thought on the subject).

CA