Try to scratch it with your thumbnail, ideally your thumbnail should skate off and make it very difficult to mar the surface of the board. If your nail digs in the blank is soft. Soft wood is difficult to work with, your tools are going to have to be exceedingly sharp at all times to fight the mushy cuts of a softer wood, making good inlets much more difficult to achieve among other things. Also, rasps will brutalize boards that are too soft.
When buying blanks, look for something hard with good grain that flows through the wrist without any runout or defects, grain runout or defects in the wrist can cause it to fracture there later on down the road.
