seat chart on iPhone
Here's what I like to do. Bring up the flight seat chart at least one hour before departure on iPhone (within an hour you won't be able to pull up the flight at all). As long as you don't close it out you can keep refreshing the seat chart. If denied an upgrade, show the GA that seat(s) are available, and point out that you know exactly which ones. Take a screenshot of chart and photo of empty seat when you get on the plane. May not get you the upgrade, but will help your arguement later.