You can check the Request.Browser.IsMobileDevice property:
MSDN[
^] which should do it, but I wouldn't just do it - sometimes you want a mobile version on a PC (to cut out the crap and get to the meat of the webpage) and sometimes you want a PC version on your mobile (because the info isn't easily or obviously available on the mobile version)
The way this site does it is nice - there is a "mobile" link at the bottom which lets me override it, and a "Full Site" link at teh top of the mobile page to get back to PC. Perhaps, if you detect a mobile and ask which they prefer, and store the result in a cookie?