With the advent of the internet, every person in the world has a bully pulpit for offering up advice. I'm not sure if other industries have the same problem publishing does, but we've got a lot of people shouting orders and issuing instructions who don't have a frigging clue what they're talking about.
How can you figure out if advice is worth the air it's shouted into?
First, remember advice can be right for one person and not another. Try something, and if it doesn't work for you, stop. Try something else.
Second, consider the source. Anyone who listens to authors tell you what works in a query isn't listening to the right source. Authors aren't READING queries. Agents, and in many cases agency interns, are.
This is not to say authors can't give you advice on writing queries. If however you get conflicting advice, remember who is actually doing the query reading when you decide which opinion deserves more weight.
Third, consider the advisor's experience, and expertise. I'm always amused to see people new to agenting offering up opinions like the Holy Grail. Saying you're an agent doesn't make you one. It's entirely possible what they say is correct but again, if you're hearing a variety of opinions, carefully weigh the experience of the person.
A lot of publishing information you find on the web is just flat out wrong. Thankfully this isn't rocket science or brain surgery and a mistake won't kill you. You might wish you were dead but you won't be.
There are no easy answers, or easy solutions. I'm wrong myself a good 3% of the time.
It's too bad agents don't come with ratings guides like movies do.
There could be:
TNTK: too new to know (much)
SBSO: Seasoned but still optimistic
HFAW: Helpful for (about) another week
JALTG: Jaded, and Looking To Gnaw