Mobile versions are a necessity nowadays. Statistically 40% of visitors come from mobile phones and 78% of the have smartphones. Mobile versions can be specified in many ways, but i have mostly used these methods:
1. Headers check for device and based on result we redirect to the correct version.
2. A responsive design with @media queries (CSS3) and with viewport metatag that combined allow you to have a responsive website re-sizable based on screens width.
Ofcourse at this points the need to validate and check your website comes up. This can be reached through mobile emulators (or from your own mobile devices, but how many do you have 🙂 ?). http://www.mobilephoneemulator.com/ is an online source that does exaclty that and since it sends the right headers it is more appropriate for the first approach. In the second case you only need to resize your browser or go to http://mobiletest.me/ that gives you ready results, this site tools can be only used for the second approach.
Last but not least if you want to find out how much compliant in w3c is your mobile site (this validation has more meaning if you use the first approach) you can go to the link: http://validator.w3.org/mobile/. Careful this validator may have no meaning for your website as the rules it uses are for old mobiles (not smartphones) and us such it depends if you want your site to appear properly in those devices.
This was a short review regarding mobile versions and their importance.
TIP: In the near future mobile versions of webpages will become not that important, and people will shift to versions based on operating systems .