Ways to differentiate between desktop user and mobile users - jquery-mobile

Question

My team at work is trying to develop a website for both desktop and mobile and we want certain function to only be available for mobile users .We have been brainstorming of different methods to detect mobile and desktop users and have brainstorm the following ideas
1) Check for screensize , if its less than certain size , we can safely assume
it is desktop
2) Check user browser user agent string
3) Capability testing
Are there any other methods to detect between Desktop and Mobile , if its possible please list the pros and cons of the method also .
Thanks

Answer 1

Intro
There are few available solutions but I will only name open-source ones, at least solutions mostly used with a jQuery/jQuery Mobile. Also be warned, this topic has the potential to start a war. On one side we have a proponents of server side detection with their community maintained databases and on the other side we have client side advocates with their browser sniffing.
Solution 1
WURFL
Solution 2
Modernizr - Server
Solution 3
Modernizer - Client
Solution 4
JavaScript based browser sniffing
<script type="text/javascript">
var agent = navigator.userAgent;
var isWebkit = (agent.indexOf("AppleWebKit") > 0);
var isIPad = (agent.indexOf("iPad") > 0);
var isIOS = (agent.indexOf("iPhone") > 0 || agent.indexOf("iPod") > 0);
var isAndroid = (agent.indexOf("Android") > 0);
var isNewBlackBerry = (agent.indexOf("AppleWebKit") > 0 && agent.indexOf("BlackBerry") > 0);
var isWebOS = (agent.indexOf("webOS") > 0);
var isWindowsMobile = (agent.indexOf("IEMobile") > 0);
var isSmallScreen = (screen.width < 767 || (isAndroid && screen.width < 1000));
var isUnknownMobile = (isWebkit && isSmallScreen);
var isMobile = (isIOS || isAndroid || isNewBlackBerry || isWebOS || isWindowsMobile || isUnknownMobile);
var isTablet = (isIPad || (isMobile && !isSmallScreen));
if ( isMobile && isSmallScreen && document.cookie.indexOf( "mobileFullSiteClicked=") < 0 ) mobileRedirect();
</script>
Solution 5
CSS Media Queries
This used to be a great and easy solution but not any more, mainly because mobile devices reached and surpass desktop computer screen sizes.
/* Smartphones (portrait and landscape) ----------- */
#media only screen
and (min-device-width : 320px)
and (max-device-width : 480px) {
/* Styles */
}
/* Smartphones (landscape) ----------- */
#media only screen
and (min-width : 321px) {
/* Styles */
}
/* Smartphones (portrait) ----------- */
#media only screen
and (max-width : 320px) {
/* Styles */
}
/* iPads (portrait and landscape) ----------- */
#media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px) {
/* Styles */
}
/* iPads (landscape) ----------- */
#media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : landscape) {
/* Styles */
}
/* iPads (portrait) ----------- */
#media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : portrait) {
/* Styles */
}
/* Desktops and laptops ----------- */
#media only screen
and (min-width : 1224px) {
/* Styles */
}
/* Large screens ----------- */
#media only screen
and (min-width : 1824px) {
/* Styles */
}
/* iPhone 4 ----------- */
#media
only screen and (-webkit-min-device-pixel-ratio : 1.5),
only screen and (min-device-pixel-ratio : 1.5) {
/* Styles */
}
Solution 6
Detect mobile browser
Never used this one personally
More info
More of this is described in my other answer, find it here.

Answer 2

1) Use CSS (thanks to Titanium for this)
/* Smartphones (portrait and landscape) ----------- */
#media only screen
and (min-device-width : 320px)
and (max-device-width : 480px) {
/* Styles */
}
/* iPads (portrait and landscape) ----------- */
#media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px) {
/* Styles */
}
/* Desktops and laptops ----------- */
#media only screen
and (min-width : 1224px) {
/* Styles */
}
2) Use JavaScript
document.write("User-agent header sent: " + navigator.userAgent);
3) What do you mean by Capability testing?