As Raje has pointed out, you can access a DIV tag from your code behind by adding the
runat="server
attribute to the DIV tag on your page:
<div id="myDivTag" runat="server" class="someCSS">
my div
</div>
Then you can access the tag from your code behind to test it's visibility like so:
if (myDivTag.Style["Display"] != "none")
{
}