Great advice already there but first of all of you haven't already get yourself a Haynes manual- we all joke about them but they do give you a good idea of whether you actually want to embark on any particular maintenance procedure before you start wielding the spanners.
There are a few ways of checking the belt tension but I think the Allen key method is probably the most popular (I favour the spring balance but probably as it was the first I found out about), but as Uptopia says there is nothing really mysterious about these motors. As well as marking the tensioners to get a rough idea I also put a little bit of white paint (or Tipex will do) on the pulley timing marks and their reference points just so it's easy to see that you're not a tooth out before you tighten it all up.
Just take your time, be methodical and check until you're happy it's right.
Another worthwhile read along with Chris Kelly's excellent info is here:
http://www.ducatisuite.com/belttension.html