You sense the presence of a specific kind of evil—that of demons, their servants, and the Abyss. The amount of information revealed depends on how long you study a particular area or subject.
1st Round: Presence or absence of creatures with the demon subtype, creatures possessed by demons, creatures under the effects of spells or spell-like abilities cast by demons, or creatures otherwise tainted by demons. Creatures tainted by demons include tieflings with demonic heritages, sorcerers with abyssal bloodlines, creatures affected by a succubus’s profane gift, creatures with demonic implants, or creatures who have the Demonic Obedience feat, and those under significant demonic influence as determined by the GM. This spell does not detect creatures of chaotic evil alignment who are not demons or significantly influenced by demons.
Additionally, this spell detects whether or not a portal or similar magical passage leads to the Abyss.
2nd Round: Number of evil auras shed by creatures with the demon subtype in the area, as well as the power of the most potent evil aura present. If you are of good alignment, and the strongest evil aura’s strength is overwhelming; if the creature has HD equal to at least twice your character level, you are stunned for 1 round and the spell ends.
3rd Round: The power and location of each aura, and what demon lord, if any, a demon is most closely affiliated with. If an aura is outside your line of sight, you discern the direction but not its exact location. Affiliation to a demon lord is only revealed when the creature detected is a demon (not merely a creature tainted by a demon). Demons receive a Will saving throw to resist revealing what demon lord they are affiliated with. If the demon succeeds at this saving throw or is not forsworn to a demon lord, you know only that this aspect of the spell returned no information.
Aside from what is detailed above, this spell otherwise functions similarly to detect evil in terms of aura power, lingering auras, overwhelming auras, and so forth.
Pathfinder Player Companion: Demon Hunter’s Handbook © 2013, Paizo Publishing, LLC; Authors: Philip Minchin, F. Wesley Schneider, and Jerome Virnich.