Should be on that back of the rims. Something like "8.5Jx18H"
edit: Oops, you asked for offset. Again, it should be on the back of the rims (either on one of the spokes or near the center). Example: ET40 or 40ET.
'02 ///M5 Imola Red / Imola Red '09 335i Alpine White / Coral Red
'04 E55 AMG Brilliant Silver / Anthracite
'96 318i Alpine White / Black
'01 330Ci Titanium Silver / Coral Red
'96 328i Jet Black / Black