The offset is stamped on the inside of the rim. As far as I know it is the distance from the mounting face of the wheel (the face that sits against the disc) to the centerline (when seen from above) of the wheel. Different manufacturers use different offsets. I bought wheels from a 3rd gen crx and despite the owners assurance that they would fit my car they actually rub on the suspension struts. You can solve this with spacers but these are illegal and have led to wheels falling off. I've heard of rollas going around with 17"s (after having guards lipped or pumped out) but not every set of 17"s will fit a rolla - regardless of whether they have the same stud pattern.
Cheers, Monty
PS - anybody want to buy some 16"s that don't fit an ae92 rolla?