To figure out Offset you can do 2 things..
1) Find the middle of the wheel. & from there measure how far the face is from the middle in millimeters. If its directly center its 0 offset. if its towards the back half its negative offset, if its toward the front, positive offset.
OR
easier method..lol
2) Measure the space in inches from the outer barrel to the rear of the face. Then use this
http://67-72chevytrucks.com/vboard/attachment.php?attachmentid=726225&stc=1&d=1302103367to determine offset!
Oh & would you trade+cash the 2 8.5s for 2 Advan Racing Oni's in 15x7+15?