This thread was good if i remember correctly.
Long story short, unless your lucky and your fingers are long enough to reach it, your probubly going to need a toy. (Preferrably one with a slight curve in it as you would for g spotting, just longer. Some longer dildos are good for this).
For me. How i found my a-spot (or Anterior fornix ergoenous zone if you want to jam it into google for anatomical images) is by sliding my two longest fingers along the front wall of the vagina making small circular motions untill you hit a small area of increased sensitivity (mine is about the size of a fingertip). Make sure she is really turned on first as that helps alot to find it.
For me there is also a small patch of slightly softer tissue that will indent differently when pressed on.
Mine is positioned centrally but hers might be slightly to the Left or Right.
Once you think youve found it, have a try with different finger motions. I find a tugging motion, as per the g-spot, works well for me.
I need the entire length of my fingers to hit it, and even then i can only just reach, so if you cant find it, chances are having a go with a toy is a good plan B.
Supersmashcache (one of the bloggers i read) also has a very nice how to a-spot and cervical orgasm guide
(As a fun aside, somep people like having the rear wall of the vagina, and various other internal spots stimulated too, so having an experiment is worth a go)