There is a vast amount of information on the web, publicly accessible. Qualified observers can assess the technical skill of a web company by looking at a site and the page construction, they can perform complex analysis using validators and sites like http://websiteoptimization.com. They can easily determine the versions of software in use (unless security was done carefully), they can check how many sites have the same IP address (same server).
Old versions of everything - site is not currently being maintained. Opportunity for maintenance and hosting upgrades. This can be very low risk if a robust application is in use, and it hasn’t be heavily customized. Same is true for copyright notices.
Broken link to web company - see above.
Poor design and application integration, broken links, missing images - site visitors have high expectations. A bad site can cost a company alot in lost business. A quick link to a demo may help the site owner see the value of your proposal.
Glaring security issues - it is easy to quickly check a site with non-destructive SQL injection testing and invalid data testing, as well as simple certificate checks. Alert the site owner with a screenshot or URL that demonstrates the vulnerability as well as a quick description of possible resolutions.
Scanning sites for information such as older versions of Apache/PHP and applications is reasonably cost effective. A few innovative email templates should allow a professional message to be delivered to prospective clients.
Finally, most sites last about 3 years before they need to be refreshed, improved, redesigned.