Great question. Do you mean informing the user when they are currently using a mobile that they can view the site in a mobile-friendly format, or do you mean letting regular 'desktop' users know that if they wanted to view on a mobile the site will display nicely for them?
If it's the former then you can either use script to detect the device (or screen resolution if you're going down that route) and then automatically display the correct version. I'm sure other people will leave more comprehensive details on this route though. I'm going to answer the question on the assumption that you mean 'how do you let a desktop user know that there is a different mobile version of this site'
Besides, with all the 'mobile' devices around these days (iPad, TVs, Netbooks, Phones, PSP...) it's getting increasingly difficult to determine what qualifies as a desktop and what is a 'mobile'. (I prefer 'Portable Device' personally)
Jakob Nielsen recommends that you provide:
explicit links from the full site to the mobile site and from the
mobile site to the full site
Some example sites that do this are: Flickr, BBC News, GameSpot, IMDB and a little known site called UX.StackExchange. You're not alerted directly that there is a mobile-friendly version of the site available when you connect, but all these sites display the information in the same place: The Footer. Gamespot, IMDB and UX.SE go one step further and actually uses the link as a link to the Mobile Site itself, while the others take you to a separate page detailing the mobile usage of the site.
Therefore, if you want to follow Nielsens guidelines, as well as to follow current conventions you should provide a link to the Mobile in the footer of the page, ideally setting this link to the mobile view itself, rather than just a content page about mobile usage.