Adding to the previous comment, to do this you need to firstly make sure that the guitar is in tune anyway.

But, if you have a pretty good ear for hearing if the two tones are correctly matched, use software called Power Tab editor. Download this from cnet.downloads.com, or search for it from Google.com

Start it up, and setup your score so the guitar is tuned down half a step. Then make it play each string open so you hear what it should sound like. Tune to this, and there you have it.

Power Tab has every possible tuning for the guitar, bass, double bass, cello, sax, banjo, acordian you name it...
I hope dis helps a little