Some, but not all, open houses are listed on Trulia. As far as I know, you can't have Trulia email you when an open house is added - alerts are only for status changes and prices changes. However, if a house in your saved folder does have an open house listed, it will appear right under the address with the word OPEN bolded.
Like the other comments state, working with an agent will allow you find out about more open houses, and find out the latest listings as they hit the market, in addition to the professional guidance an agent can provide. Let me know what questions you have and happy house hunting!