Unfortunately, it isn't possible to do so technically at this time, and honestly, it probably isn't possible to do it logistically in a lot of cases as well. Moderators don't have the same problem as the OP and we can't tell if an answer is "correct" nor is it within our job description to do so.
One of the fairly core principles of SE is that moderators are just normal users who happen to know how the site operates and are chosen (either by selection by staff for betas or public elections for non-beta sites) to help make sure the site runs well in situations when the normal community self-governance can't respond quickly enough or information necessary for the decision is considered to sensitive to be public.
We aren't better at the subject matter than any other user and can't really decide what defines an answer. It would be a gross overstep of our authority to do so. It might be interesting to have a system that would mark an answer much more highly voted than others as the answer automatically if the OP hasn't been back to the site after a long period of time, but I can't think of anything else that would really fit in to the culture behind Stack Exchange as I understand it anyway (not that that hasn't shifted in the past and couldn't shift in the future.)
As far as the suggestions of closing or tagging with a resolved tag, while these constructs work well in Meta and are often used, that has to do with the way that Meta is different from the main site. Closing a question on meta simply means that no more discussion is needed, where as closing a question on the main site indicates that the question does not fit. There is always room for an answer to be improved on or even often the possibility that things could change making an answer no longer relevant. If the question fits the site and is asked well, closure really isn't an option.
Similarly, tagging is designed to categorize a question. In Meta, the "Resolved" tag makes sense as the status of a requested change is relevant to the discussion of that request. On the main site, tagging a question as "resolved" or "answered" doesn't make sense in the context of the tagging system and, in fact, there is no special "Resolved" tag on the main site for exactly that reason.
While I do agree with you that it would be nice to be able to formally recognize good answers as "the answer" upvotes are really more important than a formally marked solution. This is also reflected in the way which reputation is granted for an answer. Upvotes for answers give you 10 reputation while marking as a solution only grants 15. The solution option is more there for the short term so that the OP can acknowledge which answer was most useful to them as answers are still close together in voting. As one answer becomes the clear winner, having a marked answer really isn't necessary as the solution that is most generally agreed upon is clear.