Depends on the colour you chose.
If it's black, the 630 and 635 use the same matte cover. The other colours are either glossy for the 635 or matte for the 630.
Well, the covers can be swapped.
Also, the 630 is dual-SIM, the 635 is not, so pop off that back cover and take a look!
Moved to 630 forum, renamed title slightly.
Yes, I missed that they had purchased it second-hand.
Also, the Lumia 630 has both Single and Dual SIM variants. The version sold in the UK is the Single SIM variant, so it would still look the same as a Lumia 635 inside.