The only result for CTRL-F "horcrux" is in a private conversation between Dumbledore and McGonagall, and it doesn't say what it is except that it belongs to Voldemort. Dumbledore does later tell Harry that Voldy achieved immortality through some scary rituals, but says nothing about the method other than that it involves a murder, so a canon-ignorant reader wouldn't be able to make a confident connection. "Horcrux" could very well be Voldermort's super-weapon, or a fancy term for "hideout".
As for clues to Q=V that don't rely on canon knowledge, the two biggest ones that come to mind are the sense of "doom" that Harry repeatedly perceives when coming physically near Quirrell (when something magical happens to Harry that is unusual or impossible even in the wizarding world, it's safe to assume that it's a consequence of his unique battle with Voldemort), and especially the tale he tells about Voldemort and the monastery, which despite his cover story of a deliberate "survivor" should make anyone raise an eyebrow.
On the other side, however, there is the fact that, in a marginally subtler way, Quirrell is NOT Voldemort. Everything we are told about Voldemort in MoR (at least part of which comes from reliable accounts) matches canon Voldemort and suggest an equally cartoonesque villain composed mostly of questionable motives, self-defeating pettiness and pointless cruelty, with zero PR skills and awful fashion sense, not to mention a certain fondness for the Idiot Ball. But if Quirrell is Voldemort, that requires Voldemort being not just far smarter and more patient, but possessing ambitions more sophisticated than being a Dark Lord on his Dark Throne in the land of Britain where the Shadows lie.
Which just so happened to have been the entire core of his character! For all functional and narrative purposes, whatever change Voldemort underwent when he turned into Quirrellmort was so drastic that we might as well say that he is no longer Voldemort.
Everything we are told about Voldemort in MoR (at least part of which comes from reliable accounts) matches canon Voldemort and suggest an equally cartoonesque villain composed mostly of questionable motives, self-defeating pettiness and pointless cruelty, with zero PR skills and awful fashion sense, not to mention a certain fondness for the Idiot Ball.
Except we also have Dumbledore describing V as clever like Harry, or words to that effect. The two monastery stories seem consistent with this: first of all, canon!Voldemort would never have sought out a ...
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.