If the roots have been allowed to dry out, they may never recover. Second, you want sufficient large roots to hold the bush in place when you plant it. Third, you want some fibrous roots that will develop feeder roots quickly. Fourth, the roots should be healthy. Healthy roots are medium to light brown or tan, and bend.
Dead roots are dark brown all the way through, and snap.
Next look at the crown, or bud union. You are looking for enough undamaged surface for new canes to pop out. You might even notice little red bumps, which are the start of new canes. Bushes grown in California and Texas will have fairly well developed bud unions, while bushes from Oregon or Canada will have smaller bud unions. Next look at the canes.