Should I upvote a question if I answer it?
Does the fact that I am answering make a question a good question?
If a question produces good answers, does that make it a good question?
Without wanting to be too unhelpful, you should upvote questions you think are good questions. One way to determine this is by the tooltip on the upvote button:
This question shows research effort; it is useful and clear
It is perfectly possible for you to think a question is not a good question ("This question does not show any research effort; it is unclear or not useful") but still one you can write a good answer to - although often these questions are duplicates of previously asked questions, so do look for that first.
A question that produces good answers is an indication that it is a good question, but I wouldn't say always a good question. At times, there can be questions which are incredibly lazily written, but a community member puts an heroic amount of effort into writing a superb answer. In those cases, it's often possible to improve the question by editing it!