We cannot do much. Unless it is Open Hardware, where you can know all design details, you can create your own CPU architecture from scratch, you can change everything and design anything. Then, you can assume that you have full control. But still, you are limited by the laws of physics. You cannot see small things, you can use other devices, such as microscopes, to display that things in a huge zoom. You cannot have objectively trustless randomness. You can only have things that are random enough for your purposes, that is all you can get. And to test that, you can get some samples and check some basic things. For example, when you roll a dice, you can check that you are not getting four every time. But if you would, could you call it random? https://xkcd.com/221/