I start from the assumption that every person posting on a gun forum is, at best, the kind of person who goes to the range maybe four times a year and shoots from a bench, because, frankly, that description easily encompasses the vast majority of gun owners.
With my initial expectations set low, it makes it easy to separate the wheat from the chaff.
Whether or not I deem a particular post or user to be trustworthy can hinge on a number of things. For instance, do they post pictures of actual range sessions, rather than just gun pr0n?
Do they talk about having attended training classes or shooting competitively? If they're a competitive shooter, are they ranked?
Do they post actual data vs. just regurgitating something they picked up elsewhere? If they do post information that isn't original research, is it sourced properly?
Are they more interested in things like aesthetics or "soul" than whether or not a particular gun works effectively?
Are they unreasonably brand-loyal?
How often do other members who have actual knowledge vouch for the posts of this person?
If their posts are on a subject I have first hand experience with, how well does what they say match up with my experience, and if it doesn't, do they give reasonable information regarding the deviation?
Do they answer questions directed to them, especially if those questions may not jive with what they've posted?
Granted, this is kind of an unscientific way of going about things, but it does a reasonable job of allowing me to figure out who's for real, and who's not.