I won't say there are any strict guidelines on how you should vote up or down. Merely some suggestions...
If you feel that a particular id-request question is good and well formed (even if you don't know what the series is), feel free to up vote it. But if you that is is not particularly good question (i.e., too vague) you are free to downvote it as such. If you do, try to leave a comment explaining why, it'll help better educate the OP and other users who might have similar questions.
Due to the ambiguous nature of identification question, many users are unsure of the quality. If you're unsure, feel free to ask someone. They'll be more than happy to help. If you're unsure how to gauge the quality of an id-request, use the guidelines as a starting guide. Does it fulfill the minimum criteria? Why or why not? Once you get a good feel for what's good and bad, things should come naturally to you. It's perfectly natural to be confused, because people might life to ride the bandwagon with voting.
When judging the quality of a question, consider the following:
Does the title adequately summarize the body? "I need help remembering this anime" doesn't make a good question, you'll need to know more off then bat before you can even start looking. If I were to ask "What is this anime with sky-surfing mech about a boy with messy brown hair that's looking for a girl with blue hair?" you have a better place to start, because right off the bat you can ask yourself, do have I seen or heard of this series?
How well does the question fulfill the guidelines? The nature of id-requests rely heavily on a person's memory, which is rarely infallible. We typically then to piece thing together from our everyday, so piece might get jumbled up. You might confuse details from a childhood TV series with one from a book you read recently or as a child. The guidelines are there as a minimum standard to help you get started and organized, whether people choose to follow them or not is up to them.
Do you understand what they are trying to ask about If can't understand what the OP is talking about, ask them or try to help them better articulate their question. Given the international notoriety of the media, it's likely that English isn't everyone's primary language.
Did the OP attempt to at least try to do some (re)searching themselves? Help vampires have plagued the Stack Exchange family of sites for a long time and are typically unavoidable given the nature of the site and community. We don't have it as bad on our site, but that doesn't mean we should be lax about it. If a question feel like they asking "plx halp me find this anime" and it's obviously that if they made the smallest of efforts they would've found their answer, you can consider down voting it. However before you do, take into consideration a person's technical skill, not everyone is technologically savvy. Fans of anime and manga come not only from different countries, but from different ages and demographics as well. A newcomer might not know what the latest most popular anime in Japan is or what they symbol their friend where when cosplaying as a ninja. If the OP take the time to explain why they are unable to find something or why they're looking for something, try to give then a benefit of the doubt. Nobody likes doing work for someone that didn't bother trying to do it themselves and they shouldn't.
"Did you find the question useful?" Sometimes an id-request question might help you find a series that you've never heard of otherwise and would actually like. Feel free to give the OP an upvote in addition to the answerer.
A blatantly incorrect/bad answer stands out as being bad the more down votes it has. A blatantly bad question is made more prominent with down votes. The inverse is true for upvotes. Use your best judgement and vote to help other distinguish the good from the bad.