Mobile Visitors to your site... how do you tell?

You can show businesses these stats. If the business owner is doing any kind of tracking at all, they are aware of at least these two programs that are included with most cpanel interfaces: Webalizer and AwStats.

With Webalizer, reporting includes
URL- Shows the URL that was requested by the user’s browser.
Hit – Every single HTTP request that your visitor’s browser submits is counted as a hit.
Page – A visitor’s request for any URL with content.
File – Every HTTP request is considered to be a file.
Visitor – Each specific IP address or HTTP cookie.
Visit – Each time a visitor accesses your website.
Host – A host is a visitor’s machine running a browser. Host and IP address are sometimes used interchangeably.
User Agent – The user agent is the web browser that a visitor uses to access your site (e.g. Internet Explorer, Opera, Firefox, Mobile, etc.).

AwStats (Advanced Web Statistics) is another popular open source website statistics program. You can find it on most cpanel interfaces and it also works with most major server platforms. AwStats collects a vast amount of information and data included from Webalizer. It can also collect information on which countries visitors came from, bandwidth usage, top 25 users, the duration of time visitors spent on site, most popular pages, computer operating systems used, the URL address from which visitors arrived, search keywords and key phrases that visitors used, etc.

You should be able to access the clients' cpanel and review and explain this information, but such extensive convincing is really unneccessary and time-consuming.

All the publicity about mobile users is generally enough, and just show them what their website looks like from your mobile phone. That should be plenty of info to make a sale. If they need more convincing beyond that, I would say you're wasting time with that prospect.