The object you're searching for is a NSMutableDictionary, that's the way of recognizing it from other objects, sending a isKindOfClass message. So you should create a predicate that evaluates every object:
NSPredicate* predicate= [NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
return [evaluatedObject isKindOfClass: [NSMutableDictionary class]];
}];