I used to go to ual.com and it would let me look at seats before making a purchase, but not anymore. Every site will let you do it post-purchase. This might be a good reason to make friends with someone who works directly on Sabre or Amadeus.
I personally use the ITN site if I can't get it from the airlines themselves. I've been able to get seatmaps for AA, DL, NW, and CO from their websites, for UA, US and others, I just use ITN (the AMEX site linked above).