Is it ok to tell the OP to accept an upvoted answer? I'm asking this because in my two last answers that where upvoted by the OPs I tell them to mark as accepted if the answer worked for them and I would like to know if my attitude was correct. I said that as they do this, their accept rate will be increased.
Accepting answers is an entirely optional activity. Consequently, asking users to accept your answer feels like rep whoring to me.
If the OP has a 0% accept rate, you can link them here: How does accepting an answer work?
The only time I do this is when:
The user explicitly thanks me for the answer, saying that it "worked great".
The user has not previously accepted any answers.
If those conditions are true, then I think politely mentioning that they can "click the checkmark next to the answer to indicate that it was correct" is appropriate.
And remember, if they continue to not accept any answers, they'll end up with the red 0% accept rate, which the community tends to ... frown upon.
I try not to badger people about accepting answers, but there's nothing explicit wrong with telling someone to accept an answer. In fact, I believe it's encouraged when a fairly new user is involved. Telling them to "click the checkmark" informs them as to how SE works and makes things better for everyone. If they don't figure it out, someone's going to badger them about their accept rate (unfortunately). Better fix the problem now rather than later.
I recommend against it.
It irks me particularly when the question is fairly new. Is there a reason (other than rep whoring) to pressure someone to accept an answer as early as they possibly can? What is wrong with giving other responders the opportunity to provide a better answer? If yours is still the only answer, you're potentially denying them the chance to learn if there are any other/better answers, since questions with accepted answers are largely ignored.
You already got an up-vote, which indicates that the OP found your answer helpful. Let them decide, unpressured, if and when they want to accept your answer as the answer. People have different reasons for not wanting to accept an answer immediately, and I don't think it's our place to try to force their hand.
If you genuinely want to help a user with a low accept rate, I'd say completely detach your guidance from your answer. Find one of their other questions in which you haven't participated, and comment there, pointing them toward the FAQ. This is a much cleaner approach, IMHO - it shows the user what the community expects, it absolves you from any rep-whoring connotations (since in that context it won't look like greedy badgering), and may (indirectly) ultimately get you the accept you were looking for anyway.
When I see a 'thanks' without an acceptance or up vote, I post this https://stackoverflow.com/help/someone-answers
But I do it when I encounter it regardless of who wrote the answer or who gets the rep. I think it's helpful. Sometimes my comment is upvoted, which I take as confirmation that it's helpful.
I also post the same link for snarky comments. Which, on the other hand, gets very mixed results, as it did here How do I edit the dynamic data in an ASPX file?
So you always take your chances in these sorts of things...
I've repeatedly answered questions on various sites and had to remind new and old users that they could accept my question, however...
I try to phrase it as "Is my answer correct, or could I improve it?" or "Hey ##UserName## have you seen my answer?" And I try not to do so when there might be a better answer. This is happened once where I'd provided a better answer and it was taken well.
Not everywhere moves as fast as SO and not everyone has time to keep checking their old answers. Use discretion, but don't 'never' do it.
I had been reading SO for some time before I started noticing that some answers were marked as accepted. So far I was not able to use it as a guide. Apparently, I don't see much correlation between the fact that an answer is accepted and its usefulness. I end up reading other answers anyway, and quite often there would be a later-posted answer that is actually more useful. So at this point I really don't care about this green check mark.
There is an exception to this rule: when a question asks about some very specific information, the accepted answer is usually correct. Typically such answers get dozens or even hundreds of upvotes, generating a lot of reputation, so it strikes me as greedy to ask to accept an answer on top of that.
On a more philosophical note, many questions don't really have "correct answers", so once again, I would rather discourage this "correct-answer mentality".
I think it's OK.
Other people make effort finding solution for someone's problem, and the OP often takes the solution and doesn't leave any feedback.
In my opinion, OP should be oblidged to make a feedback for each answer with non-negative score. Either accepting one of the answers (stating therefore, it's better than the others) or leave a comment stating that it doesn't solve the problem or solve it only partially or require the clarifications from answerer.
When there's no feedback from the OP, feel free to leave the comment.