It suffices to check the following (or to make it so, via a retag):
For migration to math.SE: The question has an MO toplevel-tags (i.e., a tag with a two-letter prefix corresponding to the arXiv math categories). Upon migration, sometimes the name changes slightly (beyond deletion of the prefix), and gm.general-mathematics would be removed but the migration still accepted; see this answer for details.
For migration to stats.SE: The question has at least one of the two relevant toplevel-tags st.statistics and pr.probability. The former is mapped to mathematical-statistics and the latter to probability. See this answer.
(Of course for both sites there are several or even many other tags that are compatible in addition and using them might be better in a given situation. The point of this answer is to highlight a quick solution that works alright for those that do not want to engage with the details of tagging on other sites but still want to be sure that their migrations arrive. Thanks to Willie Wong and whuber, moderators on the respective sites, and several others, for making this solution work.)