True enough, one way to assess whether or not there is a Mess ("Trade Fair") on in Frankfurt during your visit is to assess the hotel prices. This is a pretty good indicator. However, there is a specific website you can visit that lists all the Messe events:
www.messefrankfurt.com
Try that and then you will know.
Btw: The Best of Both Worlds is: There is a Messa event on, but the hotel prices are still pretty reasonable. Messe events can often translate into better girl selection at Palace and Oase. If the hotel prices are stable then you have the benefits and not the negatives...
ST