I understand that question can me migrated from one SE to another, and I suppose only a mod can do this. How do you suggest a migration/bringing a question to a mod's attention? Especially if it's an old question, perhaps no mod will ever notice if I only comment on the question.
At 3000 rep you can migrate questions as part of the closing process. Until then you can flag them for moderator attention and tell them you think it should be moved.
A single moderator, or five users with 3k+ reputation can vote to migrate the question (you can also vote on your own questions with 250+ reputation). You can use the "flag" button (see below the tags on the left side) to get a moderator's attention if you have that privilege on the site, while the normal voting process must happen naturally.
Note that there are a limited number of migration paths between the sites, so if the question is too far off-topic, you will have to re-ask it on the appropriate site. (This answer was written before the functionalty Chris mentions existed.)
If it's a question for which non of the existing migration paths are valid then flag the question for moderator attention (as detailed in yhw42's answer) regardless of whether you have voting rights or not. This will ensure that it gets seen by somebody with the ability to move it to a better place.
For an old question - if you have a vote then use it. The question will turn up on the 10K tools pages where others will see it and vote if they agree with you. If you don't have voting rights then a comment might be enough, after all you saw the question so others might too. Otherwise a flag is also appropriate in this case.