Noticing that someone else is a vampire is not so simple as it might seem.
Can I notice someone’s vampiric nature if they’re not using Blush of Life? No. Lack of Blush doesn’t immediately give away someone’s undead status. It may make them look ill, very pale, or strange, but it’s not a certain giveaway. In order to successfully discern if a character is a vampire it will require multiple successful Wits+Awareness rolls at difficulty 4 or 5 on related context clues. Possible clues which might prompt these rolls include but are not necessarily limited to: lack of a breath cloud when the weather is cold (only if the character is not using Blush); extremely specific Kindred-related language use (vague hints about preferring the nightlife or having a sensitivity to sunlight are not enough); a feat of strength or speed which approaches the edges of what humans are capable of performing; etc.
At Humanity 4 and below, vampires not using Blush of Life appear more obviously corpse-like rather than just pale/sick; this, combined with other context clues, may lower the difficulties of the related rolls to 2, but those rolls are still required.
If a character is using Blush of Life, no clues related to physical appearance/body temperature will be enough to provide a reasonable assumption that the character is Kindred and prompt a roll. Specific Kindred-related language use and inhuman abilities may still provide rationale for the Wits + Awareness rolls, diff 5.
(If a character breaches the Masquerade in a very obvious, undeniable way, the above rolls will not be necessitated.)
Remember — it is designed to be very difficult to be certain if a character is a vampire or not! If it was easy, S.I. would be snatching them off the streets left and right.
Can I notice by using disciplines? The only discipline powers that permit you to spot a vampire are: Scry the Soul from Auspex lvl 3; Sense the Beast from Animalism lvl 1; and A Taste for Blood from Blood Sorcery lvl 1 and its Ritual upgrade, Blood Walk. No other discipline power can discern someone else’s nature, not even Sense the Unseen.
Other methods: Humanity 5 and lower may develop some evidently unnatural traits. This could be clue enough, but do not make the assumption without asking your rp partner whether they intended it to be telling. The Obvious Predator Flaw can as well be clue enough to warrant a roll or a simple understanding. If the other vampire is straight-up Masquerade breaching, then a roll isn’t even needed. Talk with the other writer first in all of these cases.