1) O" backspacing means that there is a equal amount of spacing on each side of the wheel...example...3.5 on the front, and 3.5 on the rear.
2) Positive is more on the curb side of the wheel, negative has more on the frame side.
4) Lay the wheel flat, lay a straight edge accross the wheel and measure down to where the wheel bolts to the axle.
5) Most wheel companies tell what the backspacing is.
Example: If you have a 15 X 7 wheel with 4" backspace ...it is a 0" offset wheel. The reason is that a 7" wheel is 8" at the widest point and 7" between the beads of the tire when mounted.
Hope this was not too confusing.