Sometimes it's hard to tell whether they are original or not, nowadays they have made copies that look identically but are made of poorer quality materials.
Now it depends on which rayban model you have (like is it the wayfarer or the classic model?), sometimes it can be distinguishable.
First, it's that lomb sign, it should say Ray Ban on one side of the lens and also on the arm, then maybe the shape? If you'd know the price then it would've been easier to figure...
It's hard to tell, really, but I wouldn't bother that much, since it was a gift, you should appreciate it and that's it :)
I mean, it's not like you paid for them and the person ripped you off.
If they look authentic enough to you, and you can barely see any difference between those and original ray bans, just wear them, cause most people won't be able to tell either.