What is the property available to check if the page posted or not?

The Page_Load event handler in the page checks for IsPostBack property value, to ascertain whether the page is posted.

The Page.IsPostBack gets a value indicating whether the page is being loaded in response to the client postback, or it is for the first time. The value of Page.IsPostBack is True, if the page is being loaded in response to the client postback; while its value is False, when the page is loaded for the first time