You can check to see if a particular property is holding an Open House, by either contacting the property listing agency, or researching an particular area/city on Trullia or Realtor.com.
Usually, the Open Houses are listed on these sites.
Please let me know if I can be of further assistance.
Nancy Meier, Realtor
Licensed to Sell Real Estate in the State of Iowa
Ambrose & Associates Realtors