Try using a capture spy:
it(@"should return a JSON { result: 'ok', token: <SOME_TOKEN> }", ^{
[[api should] receive:@selector(myMethodCall:)];
KWCaptureSpy *spy = [delegateMock captureArgument:@selector(myCallbackMethod:) atIndex:0];
[api myMethodCall];
[[spy.argument should] equal:/* ... */];
});