100% sure. Well, encrypt everything in sight, and airgap network.
Or, don't put anything you don't want leaked on your computer in the first place. For some data, that works for me.
Short of that, you can't be sure. You can take precautions, you can be very careful, but sooner or later it comes down to an element of risk, versus either the loss of functionality and additional hassle/cost of those precautions. And that is a balance we all have to decide on for ourselves.