Clinically you can see if the occlusion is ideal or well balanced. Read about Andrews six keys of Occlusion. If these parameters are there then usually relapse doesn't happen. Radiographically you will need to check for root parallelism and angulation of the teeth. Again if the roots are parallel there is less chance of relapse. Some malocclusions require permanent retention. I always give permanent retention along with an Essix retainer for nighttime wear. Less cases come back with relapse.