I would recommend flightaware.com myself. I use this to determine their routings by flight number. The problem is that because WN changes their flight numbers every few weeks (drives me crazy) you can't tell very far ahead of time if it is a thru flight or not because it is some other routing with the same flight number.

If you have time on your hands you can also play the book flight guessing game by picking probable origination cities that could be a thru flight for your destination. I've done that too.