If you're not on well water, your water supply (city, co-op, etc.) should be able to provide you with a full water analysis. Ours looks like this - http://www.ntmwd.com/WaterQuality.html
and it's produced every month.
An explanation of what the important numbers means is here - http://www.allaboutbeer.com/homebrew/water3.html
And to automate your adjustments, you can plug the numbers in from the report to this - http://www.byo.com/brewwater/
and get an idea of what you're doing with your additions of water minerals.