As of today, and all of Sanderson's published works, what are all the instances in which Hoid has appeared throughout the Cosmere? If he was un-named, give proof that it was him.
In rough series order, Hoid has appeared in the following:
- Elantris. Named appearance, as a beggar, hired by Sarene to move weapons. Chapter 58. Like many of his appearances, it only lasts a page or two and doesn't stand out unless you've noticed his name in other books, but later editions of Elantris include a hidden final scene which make clear his motives are more mysterious than weapons smuggling.
- Also The Emperor's Soul, not exactly appeared, but as the Imperial Fool he is responsible for Shai's predicament. A scene was removed where he directly appeared; he appeared unnamed, but description (hawk face, black and white costume) and conversation closely matches other appearances. Confirmed along with deleted scene on Sanderson's website.
- Mistborn series, in every last work.
- Mistborn: The Final Empire. Named appearance, as a beggar, consulted by Kelsier for information. Part Three, Chapter 19.
- The Well of Ascension. Unnamed, described only as an old man, the one Elend discusses the attack on Terris with, and overheard Elend's mention of the Well in Luthadel. Part Five, Chapter 56. Other scenes implying what he was up to were removed, but are shown explicitly later. Sanderson details his presence here.
- The Hero of Ages. Named by Cett in his list of contacts, and appears as a beggar, but only from a distance, as the one Vin chooses not to contact on a gut feeling, so not much of an appearance. Also confirmed by Sanderson (I'd thought this was unnamed, apparently I wasn't the only one?) and also shown explicitly later.
- The Alloy of Law. Unnamed, appears only as a beggar, talking to the wedding couple at the reception Wax attends before it is attacked. Chapter 4. Spotted by fan and confirmed by Sanderson.
- Shadows of Self. Named, performing as one of Wax's coach drivers, possibly listening in on Wax's important conversation on that ride.
- The Bands of Mourning. Named, appearing as a beggar again outside the party at New Seran, this time getting involved and slipping Wax a clue vital to the case at hand.
- Mistborn: Secret History. Unnamed, but clearly described so as to make no mistake (white hair, angular face), gets named Drifter for the rest of the novella. Details and confirms his appearances in both Well and Hero from a new perspective.
- Warbreaker. Named, playing the part of storyteller to Lightsong and Siri, and giving Siri plenty of background on the Returned. Chapter 32. Probably the longest conversation he has before...
- The Stormlight Archive, all books so far. Gives his name to Dalinar. Hoid actually sticks around as a side character titled the King's Wit for larger portions of the plot, although he vanishes and reappears as he wishes. That's not even all he gets up to; he has just as many random or implied appearances -
- At the end of each book he manages to greet a character just returning to the plot, in a place he should have no normal reason to expect them
- There's a set of strangers to Roshar looking for him around the Purelake, for unknown reasons.
- Sigzil's apprenticeship as a Worldsinger was to Hoid
- Rock relates a tale that strongly implies (since Sigzil recognizes the description) his people know Hoid as a trickster god, and a travelling one, usually popping in through their mountain pools
- Hoid meets Shallan when she is still a child in a broken home, and has a much stronger emotional effect on her than he realizes
- Some of the chapter intros detail letters from Hoid and a reply back to him that together offer a look into some of his goals and intents (credit to @KutuluMike)
- Hoid snarks to Jasnah a random factoid that he spent a long time being digested. As a brick joke, it turns out Lift recalls in Edgedancer our distinctive strange hero jumping down a greatshell's mouth.
The Stormlight Archive probably is the clearest look at this character provided so far, giving a base physical description (angular sharp face, distinctive white hair sometimes dyed black, black and white seem to be his theme colors) and general manner (merry snark and vague artistic seriousness in random measure) that can help identify him elsewhere.
Hoid does not appear in either Sixth of the Dusk or Shadows for Silence in the Forests of Hell, the first of which is sparse on characters, the second flat denied by the author in the foreward before the story even starts (although Sanderson's assistant and treasurer of minutiae Peter Ahlstrom has joked that he wasn't convinced). Naturally, any of Sanderson's non-Cosmere works would exclude Hoid as well (Rithmatist, Reckoners, etc).
Most of this information, especially citations for when Hoid is sneaking around Mistborn, is pulled from the Coppermind wiki entry on Hoid.
In Edgedancer, while having a conversation with Wyndle, Lift refers to "Ol' Whitehair", a friend of hers that winked at her as he jumped into the mouth of a large creature. Not only does this behavior and the description of white hair sound like Hoid, but towards the end of Words of Radiance, Hoid offhandedly mentions having recently spent the better part of a year digesting in a large creature's stomach which seems to confirm my theory.