The Magic Sense adept power sounds pretty interesting, but it's written as:

Magic Sense
Cost: .5
This power grants the adept extra sensitivity to the ebbs and flows of mana in his vicinity. The adept can sense magical activity on the same plane within (Magic x 10) meters of his person. This power will detect active foci, spells, mana barriers, dual-natured beings, and mana anomalies; on the astral it will also detect astral forms. Treat this power as the Detect Magic spell (p. 199, SR4), with a Force equal to the adept’s Magic.

The Detect Magic spell from SR4 words things a bit differently on what it can detect, but this appears to be based on the idea that the spell is not limited to the same plane (despite the general casting rules) allowing a totally non-astral magician to detect an astral form spirit. Magic Sense can't do this. OK, no problems here.

However, since Detect Magic is an Active detection spell, it requires a Magic + Spellcasting roll (resisted). The problem is that (non-mystic) adepts can't get Spellcasting and it seems against the spirit of the rules to require it for the Magic Sense power. Should it be replaced with another skill (Perception?) or possibly just use Magic x 2 in a way similar to the many magical effects that use Force x 2?

Also, since this is treated as a spell, does it cause drain? Can it be sustained?

Sounds like it should be treated like a sense - ie, only works when actively used...
...But powerful magical effects would be felt, in the same way that you might catch a glimpse out of the corner of your eye even when not actively looking for something.

If it has to be actively used, I'd imagine there'd have to be a cost of some sort [from the effort of concentrating]...
...And probably use Perception for it [it's just another sense to the user].

I'd say they just get straight magic, because, they don't have spellcasting.

Isn't that called Assensing?

I generally treat it like any critter power. If the critter doesn't have a relevant skill use a stat instead, usually Intuition or Willpower..

Stuff like Compulsion, Fear and in particular Divining all use stat's instead of a skill.

I'd go with Will here because it is actively resisted, i.e. a contest of wills...

If I were to go with an attribute, I'd go with INT, cause its the base for perception.

" I feel a disturbance in the force. A presence I have not felt in some time."

" Man, I am getting a realy negative vibe from you dude!"

Always felt like it was just another copy-and-paste addition to SR4. You know, one of those "well, it's here in previous editions, so let's bring it over and not really think too much about it" being the design decision.

There's no right or wrong answer about how to implement it because it just doesn't fit the existing rules. If anything, I'd simply change it so that the adept has a better chance of noticing magic (perhaps adding Magic to test to notice), and allow such rolls whenever a suitable magical effect is in the area. That or just remove it and point to the Sensing metamagic for any players who want their characters to have that ability.

How does Magic Sense interact with Spell Resistance, Arcane Arrester, and Cloak. Each of those says that they only work against spells, but Magic Sense says to treat it as a spell, so... they work against it?

No. It says it functions as the spell, but that does not make it a spell.

Why would you treat it as a spell for some purposes but not for others?

SM, 178, "Treat this power as the Detect Magic spell (p. 199, SR4), with a Force equal to the adept’s Magic."

Because the rules say to treat it as the spell for its effect. It is still an adept power, so it it still not a spell. Even if it has the same effects as the spell.

It seems to me that this power has the same effect as if the spell was permanently sustained on the character at a Force equal to the character's Magic. So he always has the sense, he never needs to roll to gain it. Am I missing something? It seems pretty straightforward.

EDIT: Um, I mean that as an honest "Am I missing something?", rather than a sarcastic one.

You've pretty much got it Eryk.

My recommendation is to assume that he has the maximum number of hits allowed, that would be his Magic rating.

Its an active detection spell. I'd have them roll magic when they try to sense.

Active detection spells don't work that way. With active detection spells, you only roll once, when the spell is cast. In this case, there is no casting. The sense simply is. And adept senses are always treated as if they have the maximum possible rating.

Right. In which case, Its almost like a perception test. Rolled when applicable.

Actually, in a sudden dose of d'oh it occurs to me that I'm probably talking rubbish.

I just looked at Motion Sense, which is on the same darn page as Magic Sense, and it uses Perception + Magic, which makes a whole heap more sense.

You could use Assensing + Magic for Adepts that have Astral Perception when they are Perceiving Astrally as an alternative, although why you would have to is beyond me.

Maybe for things on the astral plane within range of the sense but blocked by astral shadows for normal astral perception?

Ya know, like behind a wall?

Yeah, good point.

So rolling Magic + Perception in the physical world or Magic + Assensing in the astral world seems reasonable (rather than Magic + Spellcasting)?

Sounds reasonable.

How does an Adept without Astral Perception learn Assensing?

It's not relevant - if the adept lacks Astral Perception, they can't use Magic Sense to detect astral things (requiring Assensing) with a few exceptions which would use Perception (since the adept is still physical rather than astral).