i want go from .10 strings to .13's. how will i adjust the intonation and how do i know when its right?
It will have the right intonation when the twelfth fret harmonic is perfectly in tune with the open note of the string. So, when the G string is played open, it should obviously be a G. When you play the twelfth fret harmonic, it too should be a perfect G. To change it, there should be some screws on your guitar on the saddles where the strings rest. Assuming it is a strat style, they will be behind the bridge running parallel to the strings. Adjust these so the saddle moves slightly forward or backwards until the intonation is correct. Or, you could always have a professional do it if you can't get it right. Good luck!