For example this question, macbook pro late 2011, mac os x lion 10.7.3, beach ball freezing, could benefit by being migrated to the Apple - Stack Exchange What is the suggested way to do so on Superuser? Should I flag it for moderator attention. Leave a comment stating such, etc.
Once you reach 3000 reputation points, you'll be able to vote to close questions. It takes 5 votes to close a question, and requires a consensus of 3/5 users for a migration to occur. If you're under 3k, you can do the following:
Flag the question as off topic. If the site is not among the list of common options (e.g. Apple.SE), I recommend flagging the question for ♦ moderator attention and specifying where you think it belongs.
Leave a friendly comment to inform the user of your choice to migrate. Users may think that they need to repost the question themselves but this is not necessary. The ♦ moderators or higher reputation users will take care of it.