Edit if obvious, close if not
...and by obvious, I more or less mean code-golf, as, off the top of my head, I cannot think of any other criteria tag that’s so universally scored.
If a question is tagged code-golf, 99 times out of 100, it means that the shortest code in bytes wins. Older posts may have scored by characters rather than bytes, but the general rule now is bytes, so it’s a good default. If a question is tagged with code golf and doesn’t explicitly state the winning criteria in the question, I’d recommend editing in
This is code-golf so the shortest code in bytes wins
unless anything in the question would contradict this (for example, some older challenges have "the shortest, fastest code wins", or require you to count specific characters/bytes differently).
For any other winning criteria, you need clarification. So if a challenge is tagged with (for example) fastest-code and contains no winning criteria or details on how to score answers, vote to close it as off-topic and without a winning criteria (or unclear if there is an objective criteria, but it's lacking in a couple of key areas).
Sometimes (primarily with older challenges or by new users), a challenge will contain 2 winning criteria, especially in the tags. If this is the case, I would suggest removing the "wrong" tag if the question explicitly states one as the winning criteria (e.g. "shortest code wins" but is tagged code-golf, fastest-code. In this case, remove fastest-code). If it isn't clear which is intended, vote to close as unclear.
Finally, if the criteria is not code-golf, but the challenge author includes a partial description of how to score without being fully complete, I would say that editing is acceptable, but treat carefully, and consider VTCing instead, as you may miss the original intent of the author
If the challenge author is still around, I would suggest asking for clarification in the comments first before doing either of these things. While they're both reversible, both (and especially closing) can discourage a challenge author. You can check how active a user is by navigating to their user profile (by clicking on their username) and checking the side bar. Obviously, if not clarified within a couple of days, edit and vote away.