Under what circumstances do ordinary mortals get a roll to notice that a vampire is not just "odd" but actually inhuman? When is it automatic? Aside from, say, being found standing over a body with blood dripping from its fangs. This seems fundamental in a game where maintaining a masquerade is so important, yet the core book does not seem to address it.
Page 75 of V:tR describes how vampires no longer have reflections, or are blurred (unless the vampire consciously chooses to suppress this effect, pg 170), so this would give an attentive mortal a chance to notice that there's something unnatural about a person. Indeed, the book mentions witchhunters using small hand mirrors to scan for the undead.
Pages 184-185 describes how Humanity serves as a cap on social pools with mortals, and describes how vampires (especially low-Humanity vamps) seem creepy or unnatural to normal humans even if there's no obvious sign.
Perhaps there's an expansion that covers this explicitly? If not, if this is left to the discretion of the Storyteller and players, I'll accept as an answer the best set of house rules or guidelines.
Since there are so many variables, I'd like to give this baseline: the vampire is dressed ordinarily, fairly young, and not a Nosferatu; the mortal is not already aware that the supernatural exists; the vampire is being ordinarily cautious: simulating breathing, trying not to let his or her fangs show; lighting is dim, as in streetlights or a decent restaurant. People just walking past should probably take little notice, but what if the kindred engages in extended conversation with a mortal? What if he or she uses relatively subtle disciplines like aura sight or regeneration?
As @Nigralbus points out, a person can fool themselves about almost anything. Even seeing a guy turn into a cloud of bats can be written off as a hallucination from to much cheap wine. But there has to be a limit, a point where a skeptical person will start to really wonder, and I'm wondering if the rules make any attempt to define this.