It would be a good idea to use the Firebug plugin in Firefox and load it up beforehand by right clicking on a div close to where this dynamic div gets positioned - then choose "Inspect Element with Firebug" - then perform the action that is supposed to bring up this dynamic div - Firebug will highlight the change in page content for a brief moment that should help you to locate this new div - copy either its id (if set)(highest priority) or classname (if unique)(2nd highest priority) or right click and choose "Copy xpath" (if constant for multiple invocations - could be unlikely)(lowest priority). Then you should be able to apply your requisite condition by extracting .Text from this div.
I recently used this approach to check whether a dynamically generated drop-down list in my application gets displayed above or below its button (so technically a float-up or drop-down list) in a hovering dynamic div.
There is one catch though that you might want to write a wrapper over FindElement() containing a try-catch block so that the webdriver does not break your test in case it cannot find this div. Then you can add the != null condition before extracting the .Text from the div.