When a customer endorses a professional how can you be sure it is not a relative or friend? The truth is you can't. This is why photos of the work are just as important as the testimonials are. What do the former customers say? Great price? Neat work? Quality parts? Are you sure they are saying the things that are important to you? Read carefully what message is really coming across both from the professional and their customers. Usually everything works the minute it is installed so function isn't as important as quality and craftsmanship in my opinion.
Which sites are reliable for recommendations is up to you. I like the sites which do not charge the professional or consumer to sign up. Why? Because if they do then there seems to me a conflict of interest occurs when a professional pays to be promoted as trustworthy or "Certified" without posting how when and why they are "Certified".
No complaints or disciplinary action from the licensing Governmental Agency is the best indication of a reliable and trustworthy professional. You can check the status of a licensed professional online. In California, the Department of Consumer Affairs which is the Administrator of the Contractor State License Board electronically gives you insight into the professional you are about to hire.
And finally, background checks. Sounds great right? Did you know most background checks are done on the owner of the company and not the technician who will come to your home or business. If you want a background check ask the website or company to send you one on the technician coming to your home or business. And again how do you know who the background check is on since you really don't know the person coming to your home or business. Trust your instincts! If you feel uncomfortable, call someone else. The best resource for finding someone still is asking someone you know and already trust. Even then, trust.. but verify!
Finding a professional you can trust is a wonderful thing. Please be sure to tell others so they can experience quality and craftsmanship without headache and stress! Proud professionals everywhere are littered with unscrupulous and fraudulent workers posing as a professionals. Most of the time you only find out at the end of the job. So check references for larger projects to insure you are not regretting taking the "best price" at the highest cost to you, the consumer!