best to check if the rear centre of the rim is in a straight line with the centre of the front rim.
as you said if the rear sprocket is dead straight with the front sprocket going buy eye or using a small straight edge.
This can be done with a two metre alloy straight edge and measuring off the rims at the edge of the rim at the centre of the rim.
this will tell you if the off sett is right on the rear to match the front.
I checked my bikes a while back within 2mm seems ok for a old dirt bike , maybe not for a GP bike through