You can check this site and many other real estate web sites to find Open Houses without searching through every listing. I know that my company's web site, Prudential, has a link to Public Open Houses that are updated on a daily basis.
To search the Trulia site, click on the "buy" tab at the top of the screen. A drop down menu will appear that has another tab for Open Houses.
Hope this is useful.
Prudential Connecticut Realty