Who implements?

@interface A : NSObject

- (void)doSomething;

@end

@implementation A

- (void)doSomething;
{
    //Implementation
}

@end


@interface B : A
@end


@implementation B
//May or may not contain an implementation of -doSomething
@end

/*
 * The question is: at runtime, how can I determine whether or not B gets its implementation of -doSomething from the superclass A or implements it itself.
 */