<meta http-equiv="X-UA-Compatible" content="IE=8" />
should force your page to render in IE8 standards. The user may add the site to compatibility list but this tag will take precedence.
A quick way to check would be to load the page and type the following the address bar :
javascript:alert(navigator.userAgent)
If you see IE7 in the string, it is loading in compatibility mode, otherwise not.