In a perfectly fair world you should comment saying "hey, post this as an answer" and wait for the asker to respond. This requires a level of dedication I wouldn't expect out of most users at all.
If the info is good, it should be posted as an answer. Edit out the non-question bits and post an answer containing the information. Polish it up and include any helpful information you can to make it an answer of your own, or make your answer community wiki so it doesn't look/feel like you're doing it just for the rep (or both, whatever you want).
In the end it's more important that the question is answered in the proper format, don't worry so much about who's "right" it is to post that answer. There's no reason to do the whole internet a disservice just to give the asker a chance to answer their own question. Just be sure to let them know what's up so they understand the situation and can self-answer properly in the future if need be.