If the OP puts the answer in the body of the question, or if the answer ends up in the comments on the question, leave a comment asking the OP or the commenter to put the information in an actual answer.
If that doesn't happen within a couple of days, feel free to put the information in an answer yourself. To prevent the appearance of reputation point whoring/stealing, it'd be best to mark the answer as Community Wiki. This way, at least, people can vote on the answer and future readers can find the better solution.
See also:
- Mark a comment as answer to a question
- flag a comment as "should be an answer"
- Marking comment as an answer?
- Best way to deal with questions answered by the OP in the question instead of in an answer
Of course, that all assumes the answer is worth keeping around. If the solution is something obvious/trivial like "I rebooted my phone" then the proper course of action is to close the question as "Too Localized".
