deactivation/ activation of a device

If you mean computers that are authorisedthen individual computers can only be deauthorised directly on them, but if you have 2 or more computers authorised then you can log into your account on your computer's iTunes via the Store > View Account menu option and 'deauthorise all' (which you can do once every 12 months) and you can then authorise/reauthorise the computers that you still have and need : authorising and deauthorising.

If you mean computers and/or iOS devices that are associated with your account (e.g. by using iTunes Match or automatic downloads on them, or by redownloading past purchases on them) then as long as they have been associated for at least 90 days then you should be able to remove the association via your account : http://support.apple.com/kb/HT4627

how do I check what devices I have authorised? so I can deauthorise old devices

You can't see what computer's are authorised (iOS devices aren't authorised), you can only see how many - the links in my above reply will tell you how many computers are authorised, and you can also see how many computers/devices are associated to your account