Real estate agents with access to the full MLS database can generally find out the actual selling prices of property. That is because they share the information with each other via MLS as their properties go to the closing table. If you are working with an agent to find potential flips, he or she can supply you with that information.
Going solo, without an agent, just means you will have to work a little bit harder. If your local government real estate records are online, that is the easiest way to get the information. The officer who records deeds and other real estate instruments will often show the sales price on the face of the document or in the notes. In some places, people who prepare deeds must state the sales price within the body of the deed. Another possibility is to check with the property tax assessor’s office, it might have the information you need.