The first thing they would probably say to you is "Are you a web designer/developer" and why do you think its bad. I guess you can then give your opinion and tell them why its bad and good and what they do to improve it. I guess it all comes with expereince too.
I believe the most important thing a website owner needs to know is "suggested improvements" in design/coding and SEO aspect and suggestions are no insulting either. Say for a complete website review cost $xxx amount and you do it free, webmasters will love it. (IMO)