There are programs that will tell you. I use MediaInfo and G-Spot (yes that is really its unfortunate name) on a PC at work; not sure if there are Mac versions, of them, but there are likely functional equivalents. Just drag the clip over the app icon and a window with all the data will appear.
But really, is 1080p better? It might have higher potential resolution, but only for things that have that level of resolution to human vision in reality, which most things don't. And if you are sitting 12 ft away from a 42" screen, it really doesn't matter at all. Only 1080p60 has real advantages, but those are small and 1080p60 isn't available and probably won't be for a very long time. 1080p24, the format consumer download video uses, has disadvantages that might zero out any potential advantages over 720p.
720p60 and 1080i30 have nearly identical perceived resolution regardless of what the base pixel map numbers imply, which is why no one has ever been able to distinguish a 720p60 clip from a 1080i30 clip in a double-blind test. That one format is not obviously better than the other answers why some TV networks might have somewhat arbitrarily chosen one format, and other networks have chosen the other. IOW, 1080 is not inherently better than 720.
The improved temporal resolution of 720p60 over 1080i30 is apparently enough to level that playing field, as interlace error severely reduces temporal resolution on 1080i30, and there is twice the flicker (half as many frames per second which doubles the flicker artifact level) there not to mention the addition of 3:2 pulldown judder as well. 720p60 suffers from neither of those resolution-reducing artifacts. For static shots, 1080i30 might be superior, but not for motion, and TV is not radio with pictures; things are in motion almost all of the time.
But for iTunes, 720 is 720p30, not 60, so the flicker rate (same as frame rate) is the same as 1080i30. But the flicker rate of 1080p24 is even lower (making motion artifacts more noticeable), at 24 fps. This implies that an action film might even suffer at 1080p24 in comparison to 1080i30, 720p60, or even 720p30.
Bottom line, the potential resolution (and I say potential because it does not always live up to its potential) of 1080p24 is slightly better than 720p30, but resolution is not the entire story; the increased motion artifacts on 1080p24 mostly offset any advantages, making them nearly the same. So, if you can get a 1080p24 copy of something, get it, but don't worry that you might be missing something if you can only get it as 720p30 or 720p60, because the difference is really pretty minimal, probably not even noticeable.