IPHONE - detection if the device was hacked

Question

I am trying to find that iphone / ipod is jailbreak. Then I thought that the application might try to access the file outside its scope, for example, try to find out if MobileMail.app is ...

here is the code

NSString *filePath = @"/Applications/MobileMail.app";
if ([[NSFileManager defaultManager] fileExistsAtPath:filePath])
    {
        NSString *title = @"problem";
        NSString *message = @"you are using a jailbroken iphone";
        NSString * buttonOK = @"OK";

        UIAlertView *alert = [[UIAlertView alloc] title message  
                        delegate:self  cancelButtonTitle: buttonOK   otherButtonTitles:nil];  
        [alert show];  
        [alert release];  

    }

since jailbroken iphones can access areas outside their sandbox, and a regular iphone cannot, this code will theoretically show whether the device was hacked or not, but this is a problem.

Someone suggested that Apple could not approve an application using this code, as it might seem that the application is trying to access a file outside its sandbox.

It's true? If so, can you suggest another code that could check if the application is running on a jailbreak device?

.