My question is, how do you decide what advice to follow? (assuming the different answers will all fix the problem at hand)
That's a question whose answer is important for all aspects of life, and it is made much more so in current times where nearly anyone can say nearly anything on the internet and some people will agree.
Sadly, there is no easy way to avoid every pitfall.
In reefing, this is my advice:
1. Does the advice make sense? If it does not, that may be Ok since none of us are in a position to decide if every statement is sensible. But if it doesn't seem intuitively correct (say, my current recommendation for some folks to dose ammonia lol ), then be sure to follow up by asking questions, preferably from people who you already know to trust.
2. Do you actually know anyone who is already following this advice and is your situation sufficiently similar to theirs to draw a conclusion? unfortunately, this is not foolproof because the tank you know of may thrive in spite of a detrimental practice, not because of it. But it is at least something to check.
3. If you know of someone who seems especially knowledgeable in the area the advice is give, run it by them. Maybe more than one person for complex things.
4. Don't just accept the advice manufacturers, resellers, or service companies. Treat it the same as other unvalidated advice until it gets verified somehow. Remember they may have a clear conflict of interest in getting you to buy a good or service. A nonreef example: a trail camera I just bought said to always use a certain brand of normal AA alkaline batteries. Seriously? That doesn't remotely pass test#1 above.
5. Look to the great tanks of Reef2Reef. Those that make tanks of the month, those that have been around for a long time, etc. What do they do? Beware of tanks that get set up just for photos.