Is there a way to check if there’s a hole in your roof without a ladder? Affirmative. Does the puddle only appear when it rains? Does your house get cold at night? You can’t gain “ground knowledge” (or “roof knowledge”, or whatever) but clues can be used to triangulate what’s going on.
The key thing is that “my house gets cold” is a weaker form of evidence that actually seeing the hole in your roof, so it’s a good idea to let a few clues pile up before making judgement. JREF is wrong. Sometimes a plethora of weak evidence CAN equal strong evidence.
I like to do this with internet users. I can’t look into anyone’s head and determine that they’re stupid or pathological. But there are distinctive and uniform “tells” that relate to certain types of people. I’m against making snap judgement based on just one trait, but let each additional one be a hair raised on your neck.
Pink hair: likely a woman who posts frequently on twitter about how she “needs feminism”. Like many women who “need feminism”, a cauldron of insanity, vindictiveness, and narcissism. Comments will be interpreted as death and rape threats.
Anime girl avatar: strong risk factor for trolling, being a sockpuppet, or being from /r9k/.
Self-describes as an “atheist”, “rationalist”, “free-thinker”: moderate risk factor for selective skepticism, hivemindedness, or being some social cause’s automaton (substantially more so than someone who self-describes as “religious”)
Self-describes as “traditional”: very opposed to moral degeneracy. Deletes internet browser history with the frequency and compulsion of Lady Macbeth washing her hands.
Self-describes as a good person: thoughtful and conscientious. Keeps the thermostat low enough so that neighbours don’t have to smell the decomposing hooker in the basement.
South Park Avatar: guy who’s been on the forum for 10 years, and whose every post is an impenetrable mare’s nest of inside references and in-jokes. Every reply after him being some variant of “hur hur hur, that’s exactly what you’d say, South Park Avatar Guy. Hey, everyone, check it out. South Park Avatar Guy just said the kind of thing South Park Avatar Guy says.”
Avatar of himself playing guitar: male who disregards life in favour of carefully staged pictures of himself having a life. Zoom in and the guitar will probably be strung back to front and held in the wrong hand.
PGP Key in profile description: you’re not meant to USE it. You’re meant to ADMIRE it. Woah, shit, this busta has a PGP key!
I hope this has helped you in your online adventures. More will be posted at the brisk schedule of “probably never”.