As you can tell from many of the questions here on Trulia, it is difficult for these sites to keep track of homes that go under agreement. My suggestion is to use a listing alert system from a Realtor. Most Realtors have access to this system and offer it as a free service. You can save a ton of time by creating a profile for the home that you are looking for and having only the matching listings sent to you. Once you sign on you will receive an initial list of matching homes and then you will receive one e-mail a day alerting you to new listings or price and status changes on properties you have on your master list. What makes this better than some of the big site's service is that this information is fed directly from MLS and does not rely on an agent to remember to go back to all of the sites and mark their listings as UAG or Sold. You can stop the service at any time yourself. If you have a Realtor ask them to get you set up on the system or you can use the link below to use my system. I hope this helps!
If I understand your question correctly, the only way you can find out if a house is under agreement is to consult a realtor. Whenever a house is under agreement to buy, it could be on an option to buy which means that the buyer have so many days to change their mind. It can also be an agreement pending the closing of the property. Your best way to be sure is to get the service of a realtor in your area. You can always go to open houses because most of the time the property will still be available for sale.
Lanre Bolatiwa GRI