First of all, welcome to Windows Central!
Hmm, I usually purchase my phones from an accredited retailer in a mall so chances of encountering a fake is low. Nonetheless, I still follow a safety procedure to check whether the phone is "real" or not... or just to check if the phone is fully functional or not:
1. First impressions and gut feel: That "it feels right" state; the weight, the tightness of the construction, the box... all of these should be properly executed (no loose packaging, no weird marks, no lopsided fonts, faded typesetting, etc..)
2. Down to the details:
- IMEI on the box should be the same with the IMEI indicated in the phone settings > about
- check the dimensions of the phone and see if they are exact compared with the official dimensions from Microsoft
- check, also, the weight of the phone
- check if all features (as indicated in the official specifications page) are available: ambient light sensor, accelerometer, gyro, camera OIS, etc.
- try connecting the phone to the store Wi-Fi and see if you can check for updates (fake phones can not check for updates)
- check the OS version, firmware revision number, etc. and compare it with the official listings from Microsoft.
- check the integrity of the display - if the resolution isn't really "topnotch", it's most probably a fake.
- see if the dealer would offer you an official receipt - avoid transactions with no official receipt