It is pretty simple. From the content page, you can search for the controls in the master page and if the control is found, you can do something like this:
DropDownList stateInMaster = null;
if(Master.FindControl("ddlState") != null)
stateInMaster = (DropDownList)Master.FindControl("ddlState");
staring selectedStateValue = stateInMaster.SelectedValue;