What's the aura look like on a ghoul? Does it look special/different? Do you have to assense it to notice that it's a ghoul or does it just show up like that?

It's minty green.

Otherwise it looks like other ghouls, or dual natured Infected. Seen one, you've seen them all. Really.

When you see one let me know what it looks like eh.

Since an Aura is perceived with a sense I don't have struggle to find the words to describe it, but I'm fairly certain if I had that sense, and I'd assensed one ghoul, I'd probably be able tell when I assense another one

I think that the HMHVV counts as an infection/sickness, which means that 1 hit will tell you they are sick and awakened, 2 will let you know if you've seen them before, 3 will tell you it's HMHVV, 4 will tell you they are a ghoul (assuming you know what that specific aura looks like). This is going off of the assensing table in Contacts and Adventures, which I believe is a carbon copy of SR4's table.

like any other metahuman's aura. with enough hits on the assensing test though, you'd notice that theyre diseased.

Also, theyre dual natured.

It certainly looks different (unless the ghoul uses the Masking metamagic, of course).

Runner's Companion says that the infection is always easily visible, though it does not state whether an Assensing test is necessary and if so, how many net hits are required.

Identifying that someone suffers from disease would demand 1 hit, a more precise diagnosis would call for 3, and an exact diagnosis would require 4 hits on an Assensing test.
If this mechanism was used, 1 hit would tell you that the target suffers from a disease, 3 would tell you that it's some form of HMHVV and 4 would identify it as a ghoul.

On the other hand, 2 hits are enough to leartn about the "class" of a magical target- examples given for this are Fire Elementals or Power Foci.
It would be up to GM discretion whether this includes the Infected and if so, how precise the information given is.

Likewise, the GM could decide that the changes in the aura are so drastic that they are always visible with a single hit on an Assensing test, or that they even do not require a test (though i think that would be unlikely if you need 2 hits to identify something as a Fire Elemental).

If a person goes dual natured and starts perceiving the astral plane and looks around, seeing all the auras, but they don't have the assense skill (which can't be defaulted on, if I recall correctly), can they tell that one aura is a ghoul's aura? Does the aura look any different or does it just look "humanish" like any other metahuman's aura?

I would rule no, because the way auras have been interpreted in my mind is as a multicolored spectrum. The light could mean who the heck knows what to the person seeing it, and without at least some training, what's the point?

With that being said? When I run I strongly, STRONGLY encourage players to take the perception skill at some level. Similarly, if they can view astral, I strongly suggest assensing, as it is essentially astral perception. (A good scanning program is also recommended for hackers, to cover all 3 planes on which the game operates.)

How would the scanning program be able to tell what was on the astral plane?

No Assensing skill?

p. 191 SR4A 0 Hits, Information Gained = None (Though I do think they could tell something was there.)

Enjoy.

It doesn't. It's just that as people who can astrally perceive should take assensing, people working in the matrix/Wireless should take Scan or Analyze programs, and people working in the mundane world should have perception skill.

They wouldn't. I'm just saying I recommend my players take the perception skill that fits their skill set. Having a player who can view astral but doesn't have the assensing skill is a very, very dangerous proposition with little chance of long-term survival. Maybe I should be having them use perception for simple stuff like seeing auras, but that's just my play style.