Missing components.


Hi when replacing a u2 IC on an ipad mini 2 I noticed a resistor was missing on the right of chip. "R8170" According to the Schematic this is a 4.7K 5% 1/20w resistor.
Anyway I took a quick look at a working ipad mini 2 pcb to find it was missing there too!
Without that confirmation I would have installed a resister assuming I knocked it off during repair.
After this discovery I began noticing several other places there there appears to be components missing.
I have ordered a zxw dongle tool already so maybe that can help.
My question is, if I can’t trust the Schematic how do I know whether these resistors etc are needed????
Any advice whatsoever hugely appreciated
Thank you.