- (FDModel *)_modelForClass: (Class)modelClass
    withIdentifier: (NSString *)identifier

@interface FDModel

+ (instancetype)modelWithIdentifier:(NSString *)identifier;

@end

[FDModel modelWithIdentifier:anIdentifier];

@implementation FDModelSubclass

+ (instancetype)modelWithIdentifier:(NSString *)identifier
{
    FDModel *model = [super modelWithIdentifier:identifier];
    if (model)
    {
        // do additional init stuff
    }
    return model;
}

@end

BOOL classConformsToProtocol = [class conformsToProtocol:@protocol(OKAProtocol)];

BOOL classConformsToProtocol = [self class:[OKAClass class] conformsToProtocol:@"OKAProtocol"];

------

- (BOOL)class:(Class)class conformsToProtocol:(NSString *)protocol;
{
  return [class conformsToProtocol:NSProtocolFromString(protocol)];
}