The information on this page was lovingly pieced together by Archangel Beth and put into its current form by John 'Demon Prince of Rules Lawyers' Karakash.

How can I tell whether someone's an angel or demon? Manual references would be greatly appreciated.

I'm going to concentrate on things that would reveal a mostly "hiding" celestial -- Gabriel's Malakim can make flaming hands, Malakim of David can "become immovable and indestructible", etc., which is a pretty good clue about whether they're celestial, and who they serve, and what Choir attunement they have... Though the Janus/Valefor pairings won't tell you whether someone's demonic or angelic...

• p. 82: Numinous Corpus Song of Wings -- "creates a pair of wings, reflecting your celestial heritage" could be read to reveal angelic or demonic nature when this song is invoked. (Bat-wings on demons, feather-wings on angels.) 'Course, it only works on the Singer.
• p. 87: Aura. An angel looking at an Aura-Discordant Lilim will know that he's looking at a demon (and, therefore, at which Band...). Don't ask me why -- I'd be more amused to say that anyone knows which Choir/Band pair it was, but not *which* (which would leave Lilim and Malakim as the "odd ones out" -- though a slightly-tarnished Bright Lilim could also be possible...).
• p. 94: Seraphim resonance -- with the right questions, a Seraph has a chance of spotting a celestial fibbing about their nature, their Superior, etc. The higher the check digit, the easier it will be to discover a person's true nature without getting into really nit-picky questions.
• p. 99: Elohim resonance -- even trickier than the Seraph one, but it's still possible to pick up on some things. Celestial motivations are likely to be different from human ones, and demonic and angelic motivations are likewise often different. This only works with higher check digits.
• p. 101: Malakim resonance -- demons are often selfish, and sometimes what's noble and ignoble to someone will provide a clue to that person's true nature. And a roll of 6 will pick up Balseraphs.
• p. 104: Mercurian resonance -- on things like a check digit of 3, you get "geographic and cultural origins"... This should probably call for a roll against Role, to see which "origin" gets picked up, but...
• p. 106: the Children of the Grigori "shun demons almost instinctively".
• p. 113: David's "Cold Touch" attunement can suggest whether someone is celestial or not -- by how quickly they escape. :) Also, if you see a statue moving around, there's a good chance it's a Kyrio of Stone.
• p. 114: Mercurians of Judgment probably have a better chance than normal Mercurians, since they get "a human's most deviant behavior" -- either they get celestial deviancy, or they don't get anything of that when they expected it, and can tell they're looking at a celestial. Dominic's Servitor Attunement of Heavenly Judgment can also work, if the circumstances are right, but few demons have Wills low enough to yield to the Friend of Judgment distinction!
• p. 117: Cherubim of Eli can get basic facts about the maker of an object: human, angel, or demon is always answered; more on better CDs.
• p. 118: All of Gabriel's servitors have Choir attunements that allow them to detect certain kinds of cruelty at a glance -- I would suspect that demonic cruelty would register, though not necessarily that it was demonic! Still, it's a clue. Also, if you toast someone and get 3 Essence, you just killed a demon with fire... :)
• p. 127: Seraphim of the Sword can "tell at a glance whether or not the foes they engage in battle are *ultimately* on the side of good or evil." (Emphasis mine.)
• p. 129: With luck and a good choice, a Malakite of Trade may be able to divine someone's nature by what something means to that person. Mercurians of Trade may also get lucky, if a celestial "earned" money via some unnatural means.
• p. 133: Novalis' Crown of Joy Servitor Attunement -- the duration of its effects will reveal whether the subject is demonic or not.
• p. 134: Seraphim of Yves are obnoxious... And they get bemused by people who won't shake hands with them. Malakim of Destiny "can tell a divine disturbance in the Symphony from a diabolical one." A Kyrio of Destiny who happened to possess a (likely very surprised) celestial would have complete access to the host's memories, which would undoubtedly provide *much* interesting data... Using Divine Destiny on a celestial either gets a "null reading" (it does say "mortal" in the description), or something along the lines of: "Destiny: to achieve a Word (or higher!); Fate: to be Fallen." That last may or may not show "current state," according to GM whim.
• p. 146: Habbalah may be able to detect Elohim -- they know which emotion would be worst for their subject's stability, after all. And they're more vunerable to Habbalah, too.
• p. 148: Lilim detect needs -- and if the need they detect is something like "locate and kill the demon who's around here somewhere," then she knows this is no ordinary mundane (though it might still be a loony human).
• p. 151: "Shedim have full knowledge of their hosts' thoughts, feelings and memories." If they've gotten into a celestial vessel, they'll know it. Nasty little things, Shedim.
• p. 154: "If an Impudite uses his resonance on a Cherub, an Elohite, a Kyriotate or a Mercurian, then the angel not only gets a Will roll to resist but a Perception roll as well. [...If made,] he realizes that he's being attacked by an Impudite..."
• p. 160: Asmodeus' Band Attunements permit his servitors to detect others of their own Band with a Perception roll. (And the Servitor Attunements include "Humanity," which "lets a demon pass as human under any circumstances, to anyone -- except Lucifer.") And his Captains of Integrity can detect liars or hypocrites, allowing careful questioning to spot a celestial and probably the nature of said celestial.
• p. 167: You can track down the Djinn of Belial via a permanently- attuned person's brand, if you're a Cherub, Djinn, or just know the Celestial Song of Attraction.
• p. 168: Haagenti's Consume attunement specifies "human," which suggests that one of his Servitors could spot a celestial via the "blank spot" approach. ("I can't read him. Must be celestial.")
• p. 171: If you get lucky, and you're a Baron of Hysteria, then you can tell what someone thinks would be funny -- which has a chance of being specific enough to determine nature...
• p. 173: Impudites of Fate know someone's personal history on a successful Perception roll while touching that person. Also, the Fated Future servitor attunement might catch something like "Worst Fate: to Fall."
• p. 174: Impudites of Factions might get the trust between an angel and Superior... And the amount it would take to break that!
• p. 179: Saminga's Knights of the Dead know cause of death -- such as the Malakim with the Claws Numinous Corpus who's about to step out from behind that curtain...
• p. 182: many of Vapula's servitors have little tech-toy goodies -- if you know what to look for. (But what happens if a Kyrio of Jean tries to possess the laptop of a Shedite of Vapula??)

I think that's the lot of 'em...