use a draftsman's triangle
Generally they are very accurate. You can make a simple test by striking a line against the 45 degree side then rotate it and strike another pencil line and see if they are parallel.
Set the 90 degree toward the rear and the point of the 45 into the miter guage. The "open" angle of 45 degrees is 135 degrees, if you rotate the miter head around.
Make both miters on scraps and see if they form a perfect 90 when mated together. It will either be perfect, less than 90 or greater than 90 degrees. If it's less than 90 increase the angle, if greater decrease the angle setting until you get it correct.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 06-15-2012 at 06:44 PM.