Not sure, I bet someone else will know for sure, but my rule of thumb is unless the flight is the first of the day, ala 6 am-7 am, it is a good bet there will be thru passengers on it, especially if not taking off from a coast or not completely reversing direction 180 degrees.

Took a SAN-BWI flight once at maybe 8 am once, paying for BS thinking it had to be originating but the exit rows were filled with people who started at SFO.

SW has some many wacky "published" routings of late, especially from ISP where they recently killed ISP-MDW so many midwest offerings like MCI route through MCO & TPA. Just kills me...