So I'm trying to wrap my head around this astral space thingymabobber as i'm playing a mage in a game. The rules for assensing a person are pretty well laid out. What's not laid out is, well, everything else like assensing a room, to get a feel for what has recently happened in said room. Emotions leave an imprinr: the strong the emotion, the longer and stronger it is there Lust and hate can be two strong emotions, but I can't figure out how I as the mage determine that beyond "use assensing". Or if i know someone was killed in the room, shouldnt I be able to assense the area to try to get an idea of when and how it happened?

I think what you're looking for is a metamagic called Psychometry, pg 57 of Street Magic.

I know there were some rules for astral forensics in one of the older books (Lone Star?) but not really sure how those translate to the new edition.

Take a look at the Psychometry metamagic. It's capabilities are basically improvements on Assensing, so that may give you some idea of the boundaries.

Is it different than how it was in SR3 and my mind is just playing the time warp?

Also, what about wards? I see the rules for determining the TYPE of ward (I remember seeing it in a post by whatevs, but i cant find the chart now in my book) but none for just the FORCE of a ward.

So what can you do with assensing things other than people or magical objects WITHOUT Psychometry Metamagic?

you can't. things are dead blackish shadows in the astral. Only living things have auras to assenss. Strong emptions result in background count, which you can assenss...

To lazy to look up the actual rules right now, but possibly you would be able to use assensing on "things" left behind at a crime scene, for use in ritual magic as a material link?

No, you should not. All what Assensing the area will let you know is that something violent happened not too long ago, possibly that only a few persons were involved, and a few emotional clues (like the killer(s) enjoying it a lot) but IMHO nothing more. For exemple you would not have information about how the victim was killed (shot ? Bladed weapon ? Strangulation ?), nor what the victim or the perpetrator may have looked like.

You can scan for an astral signature. SR4a p.192.

Remember that assessing is used for astral perception, so any roll to find anything in astral uses the skill.

That said, without the proper metamagic, you can't glean any deeper meaning from astral shadows other than the fact that there are there. You can read auras, astral forms, signatures, background counts, ect however.

So the Psychometry specialisation for Assensing applies only use of the Metamagic?

Ok, so killers joy is good, in your mind. What about detecting fear? And if, so, discerning where the fear is coming from, the victim (he knew he was being attacked, or he was being tortured prior to his death), or the attacker (maybe a woman was being attacked, and killed in self defense) Or maybe just shock, as the astral spirit realizes it's being disconnected from its meat body as it dies?

Out of the blue it's hard to give you a generic yet accurate statement. With enough success you can assense the residual emotions of the scene that took place and differentiate those felt by different persons that were present, but I don't think Assensing alone would allow you to connect those emotions to a specific trigger.

For exemple was the sick enjoyment of a torture session felt by the torturer, or by the sadistic onlooker of an interrogation by an ice-cold professionnal ? Was the panic and fear felt by the victim, or rather by an helpless witness who was sure to be the next, fainted out and was removed from the scene ?

Assensing can help you make sense of physical informations and get a better understanding of what happened but alone it won't be enough for your character to figure out everything.