I have seen a question on the main site that I believe should be migrated1 to another site on the network. What are the guidelines for migrating it to another site?
1 Here migration means anything from formal migration to suggesting in a comment when to migrate content to a different site.
Don't migrate crap questions that need work
If the question is in need of a lot of editing, or should be closed as "Needs detail or clarity", "Needs more focus" and/or "Opinion-based" don't migrate it. It will only end up getting closed on the target site and the migration rejected.
Informal migration is still going to cause problems for the OP if they post an identical question on the target site as it'll only end up closed again.
If informally migrating, consider linking the Tour or help/on-topic page instead of the homepage
This will make it so the user has a greater chance of finding out if their question is actually on topic there, especially if you're unsure.
In general only migrate to sites you're familiar with
To go with the previous point try and migrate to sites that you, yourself are familiar with. Not only does it create more work if it ends up not being on topic there but it also creates more confusion and annoyance to the OP who is bounced from site to site.
If you're not familiar with the target site at the very least read their Tour page and preferably their help/on-topic page so you have an idea if the question is on topic there. Ideally there will also be a post here about migration to that site.
Don't migrate questions that are on topic here
There is overlap between Science Fiction & Fantasy and several other Stack Exchange sites. A question might be on topic on another site but that doesn't mean it is off topic here. Only migrate a question if it is off topic here and on topic on the target site.
If you're not sure, don't migrate
You don't have to suggest migration and if you're not sure if it needs migration or is even on topic on the target site just don't suggest it.