Its my understanding that you only get the benefits (1st rnd detect presence, 2nd rnd # of auras, etc) of Detect Evil when you concentrate on the spell, not just cast it. Since casting a spell and concentrating on a spell both require standard actions, if you cast Detect Evil in round 1 you wont detect the presence of evil until round 2 when you concentrate on it. Is this accurate?
As soon as you cast the spell
Concentration doesn't happen only when you are injured or when you have to spend a Standard Action to concentrate on a spell. Rather, any spell being cast requires concentration, and the duration of this concentration depends on the spell being cast, it may last for a Standard Action, a full round, or multiple rounds, for spells that require concentration like Detect Magic/Evil.
From the Magic rules on Concentration:
Concentration Checks and Casting Spells
To cast a spell, you must concentrate. If something interrupts your concentration while you’re casting, you must make a concentration check or lose the spell.
Otherwise, spells with "concentration" duration would fail automatically because you did not concentrate (the Standard Action) on the round you cast the spell.
So, for example, if you cast Detect Evil, on the very first round that you cast the spell, you can tell if there is something evil within the 60 ft cone of the spell's effect. Then you can keep concentrating to try to obtain more info, or move the cone to attempt to search another area.
Contrary to the spell's description, which says that "the amount of information revealed depends on how long you study a particular area or subject.", you don't actually need to make Perception checks to notice those auras (the spell handles that). "Studying" here is talking about concentrating on the spell while looking at objects and creatures within the area of the spell.
If you decided to also attempt to notice something not obvious using a Move Action (described as "Intentionally searching for stimulus"), then you could make Perception checks to notice something else other than the auras you are looking for.
For instance, you could be trying to notice both magic auras within a 60 ft cone in front of you, and try to pinpoint the location of a sound around you by making a Perception check. Since you get to pick the order of your actions, you could attempt to find the location of a sound (say, an invisible enemy casting spells) and then move your detect-cone to that direction to see if there are any magic auras there.
To simplify: You detect the presence of auras/evil on the round that the spell is cast.
\$\begingroup\$ Thank you for your answer: Two questions though. 1st, does the concentration on the casting of the spell count as the concentration ON the spell for detecting evil? and 2nd, the first round says: "1st Round: Presence or absence of evil." So it doesnt show just evil auras on the first round, but the presence of evil altogether. So a 1HD evil aligned humanoid would register even though he doesnt have an aura right? \$\endgroup\$ Jan 29, 2019 at 21:36
\$\begingroup\$ The description of the spell clarifies that. Unless they have a strong aligned aura, like a cleric of an evil god, they are not automatically detected as evil. Only if their HD is a little higher. \$\endgroup\$ Jan 29, 2019 at 21:56
\$\begingroup\$ Can you clarify that in your answer? Also what about part 1 of my comment? \$\endgroup\$ Jan 30, 2019 at 4:15
\$\begingroup\$ @Semada to be honest, that is on the description of the spell, and not part of the question. As for the first question on your comment, that is on my answer already: " You detect the presence of auras on the round that the spell is cast." Or did I understand it incorrectly? \$\endgroup\$ Jan 30, 2019 at 10:23
\$\begingroup\$ Apologies for the late reply. Just because something is evil, doesnt mean it has an aura. The spell says you detect the presence of evil on the first round, not evil auras. \$\endgroup\$ Feb 1, 2019 at 16:25