How to tell if your key hinge (retainer clip) is broken? I own a MacBook Air 2013 and I recently broke my "comma" key. 

I had to pull it out to clean underneath and I think I was a bit rough, so one of the hooks in the key cap broke. I put it back on and it seems to work but the key pops out often. My concern is not with the key cap, I'm worried about the hinge. How can I tell the hinge is not broken? How can I know if I didn't break it either?
In researching your question, I discovered that you can purchase individual replacement MacBook Air keyboard keys, specified as manufactured by Apple.
Hope this helps.

But to answer your question, I would acquire a known good key hinge to examine and compare to the key hinge in question. This should allow you to determine if your key hinge is broken.