By knowing that they're receiving advice from someone experienced, who is qualified, and can prove it.
Because they're just other amateurs on a forum. I'm sorry, but if you just blindly accept anything anyone tells you on a forum, then that's your problem

Check people out... how qualified are they to give advice... how good are they? How long they been doing what they're doing? Are they professional and qualified, or just another amateur. I'm sorry, but if you were seeking advice on your pension, or your car, or anything else, you'd do exactly that, but when it comes to photography people just automatically assume that everything they read in a forum is true. You see this time and time again in the "beginners" forum on here... SO much terrible advice given to beginners. I actually think you should be vetted before you're allowed to respond to posts in there, because so many beginners are led down this path of gear and post processing, and never have a ****ing clue about what actually makes a good photographer in the first place it's shameful.
Of course I am - because it is simple. Check out the person giving advice is actually a k******d who knows nothing or not, just like you would if you were seeking advice on whether it was safe to take a certain drug... you'd ask your doctor, because he's qualified to give you correct advice. If you just accept something some bloke on a forum tells you, and become ill, then that's just Darwinism in action if you ask me.