Essentially, to better educate and guide users and mods (current and future), a community wiki post should be created, and any question that is successfully migrated out should be added to it. Should be sorted by Site that it was migrated to. As a bonus, a little blurb or reasoning on why it was migrated would be useful.
This is because the search feature for migrated questions (migrated:yes) is limited to the last few weeks only, before they get purged.
This will create a list of questions that can be viewed and reviewed as a guideline on if a specific question should also be migrated. This is in addition to communication between the mods of this SE and the Target SE.
Sure it adds a little work to the mods, but otherwise it would provide great insight on to why/when a question should be migrated.