The problem is more what they will do rather than what they have done¹. So to rephrase your question: “How does one prove they WILL NOT do something?”
The answer is that we can never be sure, but Adobe could increase trust in their statements by making them binding in some way. E.g. By making a legally-binding promise, one opens themselves to legal repercussions when breaking that promise, thus they are less likely do to so².
¹ Yes, Adobe is a shady company with a history of bad behaviour, but the context for the current outrage is the new terms of service.
² Yes, there’s nuance, but that’s not important for a hypothetical.
The answer is that we can never be sure, but Adobe could increase trust in their statements by making them binding in some way. E.g. By making a legally-binding promise, one opens themselves to legal repercussions when breaking that promise, thus they are less likely do to so².
¹ Yes, Adobe is a shady company with a history of bad behaviour, but the context for the current outrage is the new terms of service.
² Yes, there’s nuance, but that’s not important for a hypothetical.