The best way is to simply measure along the opposing seam and make sure the distance is the same. Since all seams should be the same length to match up without bunching, then details that cross the seams should always be at the same measured locations on each.
Some of the major pattern makers have modified their patterns to include dots at regular intervals around the pattern to make registration of these details easier.
If you want to create something similar, all you'd have to do it create a path on the edge of your seam, with a repeating shape along it, and then do the same thing on your other, matching seam, but doing something like this: