The replicas wheels fairly simple to spot.
1. The BBS logo cast into the edge is not curved to follow the wheel edge.
2. The price is often and indication.
3. The center badge design, gen BBS logos do not always fit into the center so you get silver caps and copy BBS stickers.
4. The BBS part number cast into the rim is nearly always CH025. This is for a 8x18 ET50 5x112 PCD.
5. On the smaller PCD wheels 5x100 Gen BBS should have a 56mm center cap.
6. The wheel bolt holes on 5x100 wheels will look oval.
In addition this post covers most differences.
http://uk-mkivs.net/forums/t/278376.aspx