Accesing my stuff

[Ingrim]

How do i find out if someones been accessing my couch or any furniture without me knowing?

[TristanMercer]

You can't.

What you can do though is through the menus...set the menu options itself for owner only. Then only you have permission to access it.

[Amethyst Jetaime]

If your funiture doesn't have access control, you could put a visitor list maker script in something near it.  It wouldn't tell you if someone is using the sofa but would tell you if people are coming around when you aren't there to see and give you a list of their names.

[Codewarrior Congrejo]

As the others said already you can't unless you have any script that provides you with such an information.
Closest Solution: 
Apart from an regular area scanner / visitor script;
Create a Prim, put a fully transparent texture on it and set it to phantom and for the best make it flat and place it on the floor underneath the sofa long and wide enough so that anybody who wants to sit on your furniture has to step on it or at least contact it somehow (like a carpet). Do not link it to the furniture just place it underneath.

Create a new script and copy & paste this script here inside:
http://wiki.secondlife.com/wiki/Visit_Monitor
Place your email address between the "" in    string gEmail= ""; and save the script.

This will notify you via email for everyone colliding with this prim (and not everyone just being in range). It should give you an overview of names who are getting close enough to the furniture in question and thereby assumedly sat on it ; )

I am still wondering why one would need to know who sat on their sofa, but I guess I am better off just to answer the question

[jwenting]

why would you want to anyway, yet keep the parcel open to the public?