Slight modification I am playing with. I generally let the caster detect the presence of evil* pretty handily, but detecting evil* in an individual is rather iffy.
My recently instituted house rule runs like this. When detecting evil, make a wisdom check, +1/3 caster level (or class level if a class ability), + aura strength, +2 per evil individual past the first if doing a group scan. DC 20. Success results in revealing evil in the group or individual scanned. If there are different strength individuals, do multiple rolls
Example: The PC, a 3rd level paladin with 14 wisdom scans the bar for evil. There are 5 evil 1st-level commoners and a quasit. The commoners have an evil aura strength of 0 and the quasit has 3. The paladin makes two rolls, one at +11 for the commoners, one at +6 for the quasit. If the player makes the first roll, he will note that in the bar, there are about 5 dim evil auras and a somewhat brighter one... but won't be able to discern them exactly. If he narrows his focus to one of the evil commoners, his roll will only be at +3 and he will probably miss him.
In retrospect, perhaps 15 is a better DC. I am still toying with it.
Also, as with Vaxalon, evil alignment does NOT justify attacking someone.
* - Read this as $ALIGNMENT, natch.