ward lab is the place to go. www.wardlab.com . very cheap and quick. they will even do the analysis before they were paid which i though was great.
I have heard that phoenix water is some of the worst in the country. I would start with your town or county's water analysis though. It is free and may give you all the information you need. What town do you live in?
Assuming your RO system is working fine, shouldn't you already know what your water profile will be? RO should remove most of your minerals and impurities. Or are you seeking verification of this?