1. Hiring process focus on hiring someone you can trust. If you cannot trust someone who will join your company, then there is almost nothing you can do and still remain a healthy (mental and cultural) environment to work. Lack of trust is a very very dangerous thing happening inside teams and recovering from it takes a lot of money and effort.
Also trust starts first from leadership. If you/your leaders/managers are not trusting their employees they cannot build trust between themselves. So please hire people you think you can trust first so that you will not spend most of your time imagining what might go wrong.
2. Close relation with your colleague. If you will create an environment where you have a close relation with your colleagues then it is very hard for them to cheat and will probably not think about doing it.
On the other side let me ask you back another question: Does it really matter who is doing the work (your employee or their hired "assistants") if the work done is qualitative and solves your problem? Imagine that instead of employees you will have contractors?
I'm thinking about sensitive data floating around and state agents targetting my organisation to discover flaws in design, or adding backdoors, these kind of things