The following line will provide the useragent from the person accesing your web page

 HttpContext.Current.Request.Headers["User-Agent"]

 it would look similar to the following:

useragent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.1.9) Gecko/20100315 Firefox/3.5.9 GTBDFff GTB7.0 (.NET CLR 3.5.30729)