I think most spam is easy to spot. It's very in-your-face and often involves one spammer posting huge incoherent "messages" (often with some kind of splashy visual and always with links to one or more vendors) all over the place. But if I remember right, the line is usually drawn at any post with a link to any other site that seems to be selling something. But maybe if the linked site primarily does something other than sell (and/or solicit donations)? I guess that's where my expertise runs out and the mods might be able to clarify.
Love means never having to say, "Put down that meat cleaver!"