How do I find out about the shutter speed of frames in a video?
If the data was recorded by the camera as EXIF MetaData, Adobe Bridge would reveal the camera settings under the Metadata Panel. You may have to customize within Bridge WHICH metadata you want to look at; because there are literally hundreds, thousands, of types of EXIF data; which could or could not have been embedded; AND at times; you will find what you need hidden in a strange EXIF container. 

Simply put; if you don't see a shutter speed setting under MAIN, or CAMERA metadata; the brand camera/type, could have written it elsewhere; so show all; expand all, and scroll down to see if the data is possibly there; hidden in a collapsed field. 

If you shot the video at 24fps; you are right in thinking that it likely was recorded at 1/48 / 180 shutter. However; I would think; just based off the engineering / camera / lens type which is built onto the iPhone 7; and how I would engineer the camera to work using, which it does by default-  auto-exposure; that it would rely on aperture priority for exposure; and would thus constantly adjust your shutter speed on the fly based on it's exposure method. So if you did a constant shot from a dark area to bright area; you'd literally see an on the fly adjustment from 1/48 all the way up to 1/4000 or higher. Then aperture might kick in, as the phone as no ND filters to fall back on. 

I only add that because usually iPhone footage shot outdoors in bright sunlight looks like it was recorded at HIGH shutter (strobe effect) rather than iris down. The diameter on the lens of an iPhone 7 also would make me think they would use Aperture Priority (constant) as it's practically a pin-hole camera to begin with. 

Cheers!