Or, more importantly, how long they've been around, the length of post and the time that has gone into it. For example (since he's on this page) Harry Tolen has been around since 2000 (so he hasn't been chased away, as trolls or those who offer bad advice consistently tend to be chased off or banned), in that time he's made over 2000 posts (so he is active, but not posting a ton of useless stuff), and he wrote 3 paragraphs of more or less gramatically correct and properly spelled advice (as close as you can expect on a forum).
These are usually the signs of someone who can be relied on, or at least one component to knowing the reliability of a poster on the forum. Short, poorly spelled posts, devoid of shift key useage, absent of content (cf. posts 17 through 19) tend to be indicative of one who is not suited to providing advice. Am I saying that stingray is a troll? No, but he needs to learn to post advice properly, and he needs to learn to be critical of information, and to actually read that posts that he's copying (and reference the author of the article), before he will gain much respect on these forums.
These forums were described (paraphrasing Wetman NY's site
www.skepticalaquarist.com) as a "group of self-critical aquarists". I can think of no better compliment to offer an online forum, it's what brought me here initially, and it has been shown to be true time and again. Note that "self-critical" extends beyond the fact that those who have been here for a while are always making sure that their knowledge holds up to the most current information and science, but it also extends to having little tolerance for those who do not.
Yeah, a little off topic, but hopefully it will help Gonefishing decide who is providing him with the better knowledge and help stingray become a more competent and "self-critical" aquarist, or simply decide that a "self-critical" board is not the place for him.