Photos.app: Unique smart album criteria for identifying iOS screenshots?
Screen captures do not have a camera on the metadata. You can use that to weed out most of the pictures as all pics taken with the integrated camera will state the iPhone model (and even if taken with the front or rear camera).

Second, as @bjbk said, all screen captures are .pngs.  

I tried a text filter for each resolution, including one for landscape and another for portrait orientation. I couldn't reproduce the behavior described here, which states that it does not matter the order of the dimensions. You will have to track down each dimension pair relevant for your case.
Moreover, I couldn't catch all screen captures when adding this filter; in a seemingly random fashion, some pictures were correctly selected for each resolution pair and others with the same resolution didn't.
I checked the metadata on Adobe Bridge to find out what was the difference between images but could not find any.
Also important, the dimensions of the screen capture differ from the dimensions of photos taken with the device: 

iPhone 6:


Screen captures: 1334 × 750
Pictures with rear camera: 3264 × 2448
Pictures with front camera: 1280 × 960


So that won't be a problem for filtering; anyway, the Text filter did not work as expected.

All in all, a filter made by selecting Filename - ends with - .png AND Filename - starts with - IMG_ AND Camera Model - is empty gave me the best results.

I didn't try regex because I can't get my head around it; I've had some success following guides when I find which version / syntax is appropriate for the case at hand and a good guide, but I don't really understand it.