Testing your fabric for shrinkage before you cut out your fabric with your sewing pattern will help you determine how much the fabric will shrink. Some fabric, especially synthetic blends, may not shrink at all, were as other fibers, such as silk, cotton or wool, may shrink considerably.
Testing your fabric before cutting will help you understand if you should 'grade' your pattern up to accommodate shrinkage (industry practice), or if you should wash your length of fabric before cutting. It may add an extra day in to the sewing process, but it is a worth while step. Fabric that you don't test, that possibly has a high shrinkage percentage may result in a finished garment shrinking after the first wash and not fitting correctly.
A quick and easy way to rest you fabric is to cut a piece from the corner that is approx 12cmx12cm. Keep the selvedge on, as this will help you understand the warp direction of the grain (lengthwise) and the weft direction of the grain (across from selvedge to selvedge.
Next mark with a permanent marker a perfect 10cmx10cm square. Following the manufacturers washing instructions, wash the swatch and let it dry.
After drying, press/steam the swatch.
Remeasure the the swatch on the warp and weft lines to determine the shrinkage amount. You may find that one grain shrinks more than the other.
Don't forget that this is only a 10cm square, so for example if you have a swatch and in one direction it shrunk by 1cm, this is 10%. If your pattern piece is say 70cm long, this shrinkage ratio will be 7cm. In this instance it may be best to pre-wash and press your fabric before you cut and sew.
Also keep in mind, some fabric can continue to shrink!