At present there is no mechanism, but the way in which you ask this question suggests we might be able to create one. For instance, we could create (say) a canonical-question tag and, as a community, make sure it is applied to such threads. (Tags do not apply to individual questions or answers.) Thus, a search for keywords that include "[canonical-question]" would reveal only such threads.
I have tentatively created the canonical-question tag, provided a Wiki excerpt, and applied it to two obvious threads. In doing so it occurred to me that a good use of this tag would be to identify threads that have been "buried" because they have gone unrecognized and therefore are not highly upvoted. (There's little need to tag the most highly voted questions, because they're easy enough to find.)
One can envision the need to establish clear standards for applying the canonical tag so that we don't have to discuss each case here on the Meta site. In particular, it's not difficult to imagine individuals applying this tag to threads only in order to gain attention to their posts. Among the criteria we might want to discuss are:
Whether and to what extent to use upvote counts.
Whether controversial posts--as evidenced by downvotes or extensive discussion in comments--should be made canonical.
Whether to pay attention to anything other than standard indicators of post quality or popularity (voting, bounties, and views).
One possibility is to make threads canonical only after they have been linked as duplicates to at least one question.