How do detect Android Tablets in general. Useragent?

Mo’ better to also detect “mobile” user-agent

While you may still want to detect “android” in the User-Agent to implement Android-specific features, such as touch-screen optimizations, our main message is: Should your mobile site depends on UA sniffing, please detect the strings “mobile” and “android,” rather than just “android,” in the User-Agent. This helps properly serve both your mobile and tablet visitors.

Detecting Android device via Browser

 < script language="javascript"> <!--
     var mobile = (/iphone|ipad|ipod|android|blackberry|mini|windows\sce|palm/i.test(navigator.userAgent.toLowerCase()));
              if (mobile) {
                  alert("MOBILE DEVICE DETECTED");
                  document.write("<b>----------------------------------------<br>")
                  document.write("<b>" + navigator.userAgent + "<br>")
                  document.write("<b>----------------------------------------<br>")
                  var userAgent = navigator.userAgent.toLowerCase();
                  if ((userAgent.search("android") > -1) && (userAgent.search("mobile") > -1))
                         document.write("<b> ANDROID MOBILE <br>")
                   else if ((userAgent.search("android") > -1) && !(userAgent.search("mobile") > -1))
                       document.write("<b> ANDROID TABLET <br>")
              }
              else
                  alert("NO MOBILE DEVICE DETECTED"); //--> </script>

@Carlos: In his article Tim Bray recommends this (as does another post by Google), but unfortunately it is not being applied by all tablet manufacturers.

... We recommend that manufactures of large-form-factor devices remove "Mobile" from the User Agent...

Most Android tablet user-agent strings I've seen use mobile safari, e.g. the Samsung Galaxy Tab:

Mozilla/5.0 (Linux; U; Android 2.2; en-us; SCH-I800 Build/FROYO) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1

So at the moment I am checking on device names to detect Android tablets. As long as there are just a few models on the market, that's ok but soon this will be an ugly solution.

At least in case of the XOOM, the mobile part seems to be gone:

Mozilla/5.0 (Linux; U; Android 3.0; en-us; Xoom Build/HRI39) AppleWebKit/534.13 (KHTML, like Gecko) Version/4.0 Safari/534.13

But as there are currently only tablets with Andorid 3.x, checking on Android 3 would be enough.

The issue is that the Android User-Agent is a general User-Agent and there is no difference between tablet Android and mobile Android.

This is incorrect. Mobile Android has "Mobile" string in the User-Agent header. Tablet Android does not.

But it is worth mentioning that there are quite a few tablets that report "Mobile" Safari in the userAgent and the latter is not the only/solid way to differentiate between Mobile and Tablet.