You may wish to take a step back and consider what you're actually needing to know.
How mobile the device is? Is that information in some way useful? I'm sure it could be useful to check whether it's a touch screen or a pointer device, but how mobile it is - I'm having a hard time imagining how that inormation could be useful. Thinking that on a mobile device you can't use Flash? Check for the availability of Flash directly, etc.
I suppose if you're not too familiar with HTML you might be interested in the screen resolution, but my laptop screen has about the same resolution as my wife's tablet, "mobile" wouldn't tell you what you wanted to know.