You can find your iPhone X's serial number by clicking the Print invoice link on your Apple orders page.
You can then get the manufacture week using this website: http://www.chipmunk.nl/klantenservice/applemodel.html
My 256GB Space Grey was built in week 41 (9-15 October) at factory F2 (China, Zhengzhou - Foxconn)
My 64gb Space Grey was week 42, factory C3!