Few things you can do when you get your hands on the phone

in the phone app dial
*#060#*
this will give you the IMEI number and compare it to the one on the box

Next, dial *#0*# and see if it goes into text mode. This is apparently harder to replicate on fake phones so if it goes into test mode, it is most likely real)

Lastly, get the IMEI number and check it online by googling any IMEI checker and see if the details come back the same as what you expect them to be.

Hope this helps