Depends on the type of crack, but a hard flexing is a good place to start. If it's cracked across the grain, which it usually is behind the point, the point will likely break off. If it occurs higher up the arrow, the arrow will likely break in half. Better you do it by flexing than by shooting.
Good idea to sight down the arrow if you suspect it might be cracked. If it is, it might have a noticeable kink/bend, behind the point or wherever the crack occurred. Further, there might be a slight break in the finish. It would look like a small line across the grain. Cracks that go with the grain usually open up and are easy to see. Twisting will show those splits, as Rufus points out.