There is no reliable detection method, and, we're discouraged from discussing details of specific j'broken schemes here.
See this other thread and the Apple specialist answer. No way.
quate: "There is not. Consider that even if there was, the device has already been jailbroken, meaning arbitrary code execution is possible, and the jailbreaker would just modify whatever method of detection you would use to signal that the device has not been jailbroken."