The thumb rule of watching video on a big screen and be able to make difference between FHD and HD is that the viewing distance has to be within 1.8 times the screen diagonal length. So in case of
@ISO for a 40 inch screen the max viewing distance should be within 40 inches x 1.8 = 72 inches = 6 feet.
In my panasonic 50 incher plasma (9 years + and still strong with awesome blacks even now) I sit 7 feet away for the same reason. But as
@MaSh and
@skumar has said, if its a true 720p and your TV upscales to 1080p it will be a difficult competition against the true 1080p of the same title. Yes there are some possible areas where you can catch differences for example while looking at colors or color gradients like radiating lights from a light source (look for abrupt bands as intensity decreases radiating out), if for example 1080p was 30 fps and 720p was 24 fps so panning shots / fast moving shots can at times go for a toss. But why spent so much time on finding faults, rather enjoy the movie