Sometimes you really need to know how much stretch a fabric has.  Maybe the pattern tells you to work with a particular percentage of stretch, or if you're using two stretch fabrics together, it's generally going to be easier if they have the same degree of elasticity.
 
Here's a folding ruler you can pop in a pocket when you're going to the fabric shop.  Simply unfold it, hold the fabric at one end and pull from the end of the "Unstretched Fabric" guide.  How far you make it along the other leg of the ruler tells you what percentage of stretch you're working with.
 
Available in cm or inches.
 
Designed by Jen Hogg and made in small batches in Scotland.
 
If you're buying from outside the UK - please buy through Etsy to have the correct tax applied. This should save you paying any local handling charge in addition to any tax.