Reviewing designs and work from developers starts with reviewing if everything is there what needs to be there. Is it clear what the user needs to do, are there edge cases that need to be handled, and so forth. Besides that, a designer thinks about the visual part of the feature to build.
I’m not a designer, but I worked with a couple of brilliant ones and they taught me some bits to look out for. These three will get you started:
Be keen on alignment. Margins and paddings need to match and be applied consistently. Text should be horizontally and vertically aligned within their labels.
Are things looking crammed? Almost every design benefits from adding white space. If elements are too close to each other, there is more room for the user to be confused. With added whitespace you automatically gain better usability on mobile.
Consistency is key. There should be a pattern in the fonts, font sizes and colors used.