If need be is there by way of logic and testing a way to find out or at least estimate if there are hardware backdoors on the device.
My understanding here of a backdoor is anything inside the electric/logic scheme of the SoC designed with the purpose to coduct an attack.
What this question is looking for is at best a most comprising list of concepts/approaches that have been used for testing against backdoors in hardware.
Remarks: I am aware that the question appears hilarious and that by the nature of being very very very small structures any looking into the device is rather impossible. Yet for instance in another case of Blue Pill rootkits some people suggested some concept for testing which involved executing code that would by design demand a certain predetermined duration to be executed which can be compared to extern stop-watch measured time source for comparision. So maybe there it has been thought of to have a set of testing applications that have expected results by way of the specifications stated by the hardware (i.e. that the CPU type is ARMv7) and which can be tested. I do not want to rule out any backdoor but find a way to at least reduces risks and gather more Security. I did not intent to showcase the AllWinner as something I expect a backdoor. But I have been told that the hardware they produce is open source and hence expect i.e. better backdoor testability.