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