Play an hour of video on your iPad while tethered to your iPhone. When about an hour later you get notices that you've hit 90 percent of your data and then gone over -- and you have 4GB of data -- you're probably in HD.
Yep, had that happen. No big deal on an Apple TV to get the best quality, but damn I wish the iPad app had an easy way to select quality instead of requiring us to go to the website like it's 2009.