I want to purchase a LG G3 mobile in used condition. I have seen many mobile but i feel on those mobiles something wrong so I just want to know how we can get idea about mobiles like it is original or refurbished or something fake etc.?
There should be better ways to do that, but the most precise that I see is installing in your PC the USB driver for the device, and check if it is recognized once physically connected. I've developed some time ago an application for my Samsung, and I was able to communicate with the mobile just after installed the manufacturer driver. For your case, I presume you should to download of that executable: https://www.mylgphones.com/lg-android-usb-device-drivers
As I've certainly said before, I do not advise anyone to purchase an used mobile, since as was already published somewhere, these are one of the electronic objects that more accumulate bacteria, and unless they have been properly sterilized, your health will be at risk, depending on who used it before.
I can't see how a consumer could check if a mobile phone is original based on this information.
A fake model could replicate a valid IMEI took from a lawful device.
Correct. If you are buying a second hand, always choose an item with complete package like box, manual and accessories. Check out the IMEI number of the phone if it matches in the IMEI on the box. If you are still in doubt, you can always settle for brand new unit.
It's really hard to tell. My cousin was a victim recently. He thought it was original, he said he checked IMEI, but it was still fake. He knew when he compared it to his friend's.