Start with the Internet. Search for window contractors in your local area. Also search for a reputable third-party company that has a trusted network of window contractors who have passed background checks and can provide sound references. Avoid freelance window contractors unless they can provide validated credentials and sound references.
Use Angie's List. This site allows you to review potential contractors by checking their credentials and reading reviews posted by other homeowners who have used their services before.
Go to your state's Contractor License Board website and research potential candidates there. The state government can track the reliability and reputation of contractors.
Call a local handyman and ask him for direct referrals. People in the home improvement industry can often give you unbiased and free advice.
Canvass your neighborhood. Knock on the doors of neighbors that you know recently installed new windows and ask them for a referral.