It depends in what field you are looking.
Accounts: Check the Report a Scammer archives and search their name. If you find any scam reports regarding recoveries, you have your answer.
Gold: Use a trusted middleman. You can check their vouches for the amount of times and value that he was trusted with. These are the vouches where people include "I went first". Some people don't even mention that, but you can't base yourself off of those vouches because there is a possibility they weren't put in a position of trust.
There are many trusted gold sellers as well so you won't need a middleman. The biggest problem here is impersonators but if you follow my last point you should be set here.
Services: Check their vouches AND the report a scammer archives. Search their name, if you find anything related to bad trading practices or bad service reviews, move on to the next.
Many people have a lot of vouches, but many of them also have low quality vouches. I would not hold those into account (account rentals, fire capes, etc).
Also read the threads posted by Gladiator above. ALWAYS, always always always request a PM from the person you are dealing with and have them include their UNIQUE DISCORD ID.
Compare it to the discord you are talking to and you should be golden.
Best of luck!