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.