Open IIS 7, click on your site. In the Features View, double-click on HTTP Response Headers icon.
Now click on the Add link in the right navigation pane. For Name, enter something that is meaningful to you. In my example, I chose ServerName. For Value, type in the details. I entered WFE1 in the example below. You don't need the whole server name, just some hint that will quickly identify the server.
Using Fiddler, you can easily see the result under the Headers tab.