I have an Xparts bumper at the minute...the bottom valance is integrated, and the alignment underneath the wings is very good, but not perfect. Foglight grilles are pretty much totally vented, whereas original ones are mainly solid?
However, my upper mouldings came in two pieces (for headlight washers) but I replaced them with genuine one piece mouldings instead. I also have a genuine GT centre grille.
The average bloke wouldn't know it wasn't an original part.
Hopefully this helps you.
EDIT: Sorry I just re-read your post and assumed the bumper was already on the car! Part number and integrated bottom is the telltale signs.