The following 1 minute video shows you a quick and easy way to determine the stretch of a fabric.
Note that stretch is predominantly determined by 2 things:
1. Fabric knit (remember "knit one - pearl one") - ie, the way the textile is compiled (generally wovens do not stretch unless elastane is added specifically for that purpose).
2. The composition of the fabric - largely the addition of elastane.
The "memory" of a fabric refers to it's ability to bounce back to it's original state after being stretched. Fabric with elastane generally has better memory - a cotton tee shirt, for example, will often loose it's stretch recovery (memory) over time as the threads wear.