It is never a given because aircraft can be swapped if there is a mechanical or other issue. If it is a Delta mainline flight I have found using Flightaware to be helpful because there is a link you can click to see where the plane is coming from. I will also look at Delta.com on the day of the flight and work backwards based on gate assignments. For example I could see my DCA-JFK flight previously operated CVG-DCA and MEM-CVG (was having trouble finding out stuff prior to it getting to MEM) which allowed me to track the aircraft