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To find a broken flex you will need to obviously focus on the flex cables and areas of flex cables that were touched during the previous repair. For Xperia Z1, the problematic cable is the large flex cable that leads from the motherboard to all the buttons and the screen.
You will need a keen (or experienced) eye to spot a damaged flex cable by eye, and therefore some magnification is recommended.
The flex commonly tears near a bend in the flex or in a narrower area. See the pictures from two broken mic flexes for Z1. The first image is the original flex which I accidentally tore and the second is a replacement which arrived broken. Not trusting the second replacement to be ok, I repaired the original...