2-1 (ABC) & 5-1 (FOX) are always broadcast 16:9 720p.
11-1 (NBC), 17-1 (TBS), 30-1 (PBA), 36-1 (WB), 43-1 (UPN), 46-1 (CBS) are always broadcast 16:9 1080i.
Anything else right now is always broadcast 4:3 480i, such as 5-2, the SD version of the HD 5-1 broadcast.
Now, the program you are watching may or may not have been recorded using HD cameras. Is 4:3 vs. 16:9 a valid decision maker for originally SD vs originally HD material? I don't know about that. I'm sure there's plenty of movies and shows shown 16:9 that are just upsampled DVDs or something to make the 720p or 1080i broadcast signal.
For your purpose, I would think FOX would be the easy thing to look at. 5-1 is what you get with OTA antenna. 5-2 is what you would have gotten with locals over Dish (I would bet FOX local over Dish would look more like 5-2 than 5-0).
This then moves on to the other side of the equation, what your tuner box does. My LG LST-4200A is very good for determining what I am getting. There's a native mode, and I can simply look at which format light lights up. I also have both component and s-video connections. And what exactly I see changes depending on the output format setting I have selected.
And in your case, your TV may be further manipulating things. I use a Infocus 4805 projector, which can tell me onscreen the incoming signal format, and offers me a variety of self-selectable aspect handling options.
So 5-1 is always going to be 16:9, but if you have your tuner set to native or 720p output and you're watching the s-video, you might be getting a 4:3 480i picture. You tuner or your TV could be zooming in the picture if the program material is only 4:3 (so bars on the side, or in the case of my 27" TV, "bars" all around).