In light of the recent question posted (Self Evaluation) it dawned on me that a lot of questions have answers and just aren't marked because it was a one-off visit by someone.
Are moderators perhaps able to mark an answer as valid, maybe if it has 5+ votes or 10+ votes or something? Maybe even set it up that if gets over 10 or 15 votes or something it automatically gets marked as the answer?
The correct way to "mark an answer as valid" is to up-vote it. An up-vote indicates that the answer is useful or well-written or whatever other criteria you use to determine that it is a good answer. However, accepting answers is strictly the domain of the original poster.
From a similar question on MSO:
The "accepted answer" feature was never intended to mark which answer is best or even if the answer is correct. It is, simply stated, the answer that the original author found most useful in solving their problem.
… The "accepted answer" is all the original author. If you want to let [other] users also select the "accepted answer", the purpose of having a selected answer becomes redundant and useless. That decision has to come from the author, or you're just conflating the two features.
