I thought I'd try an image link from Flickr, as I'm not able to upload to DCW:
As Jediboy says above, you should see a circle with a line through it, as I have pointed to in this image. Unfortunately different lenses have bigger or smaller diameters, and therefore different thread sizes, so you may need filter(s) for each of your lenses, for example my walk-about lens is an 18-200mm which has a 72mm thread and my 100mm macro has a 67mm thread.
Regards
Nick