How do you insure the backspacing is right?

How do I know what backkspacing to get? I want 19x9's in the rear and 18x8's in the front on an 86 iroc z. How do I know what to get so that it looks right?.....ya know nothing sticking out past the fenders, rubbing anything etc.......... Is there a sure fire way to know? I'd hate to spend loot on a set of wheels that look "lowrider" or just plain don't work with my car..........Thanks for the help-