I've read the matrix rules a few times. I've seen lots about the GOD and hosts detecting you and launching IC. What I have not seen is any rules as to how to be spotted.

If I'm doing attack or sleaze my score goes up I understand, what does the GM roll against me to catch me out?

Every time you perform an attack or sleaze action, it is resisted. Each hit (not net hit) that the GM rolls adds 1 to your Overwatch score (OS)

Once your OS is 1 or higher, every 15 minutes you are in the Matrix it increases by 2d6.

Once it hits 40, GOD automatically converges on you, there is no roll - and you take 12DV matrix damage, then you have to take dumpshock of 6DV (physical or mental depending if you were in hot or cold sim), you reboot, and they alert the local authorities to your location.

That's GOD, btw, not Hosts.

Are you also asking how Hosts spot you?

In SR4 the GM would roll to beat your stealth rating. Once it got more
Hits than that rating you were spotted.

What's the opposed test when an attack or sleaze action is taken inside a host and by the GOD?

That would be Hide vs. Matrix Perception.

Where do I get the dice pools? I feel like I've missed a page with these specifics

Look at page 238 to 243.

Those are all the Matrix Actions.

Any of them that use Attack or Sleaze as a limit are illegal. Each action lists what dice pool the target resists with. Each time the GM rolls a resistance test against an attack or sleaze action, he should keep track of the number of hits he gets.

Once he gets to 40 (and remember each 15 minutes you spend in the Matrix after your first Sleaze or Attack action adds 2d6 to the total), GOD spots you automatically, no die rolls, and you get screwed.

Now, with regards to the Host spotting you, if you perform a successful Attack action, it automatically alerts the Host that someone is there, and the Patrol IC that it is running has to make a successful Matrix Perception test (see page 241)

If you make an unsuccessful Sleaze action,you automatically get a Mark on you, and it knows where you are.

Apart from that, most of the time if people want to spot you, it is Matrix Perception.