Open the back cover of your LG g3 and check all the screws. They should be in perfect alignment and you could also see D850 or D855 written on more than 2 places. If all checks great than compare the brightness with S4 and S5, counter check the number of DPIs on the official site. They all should match. Remember G3 had the highest number of dpi than Samsung S5 or S4.
Other than that there is no way u can check for refurbished one. I check these things which I have mentioned in G3. And found all of them opened with screws at misalignment.
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