In general although, how do you deal with this kind of prove a negative situation? Hire high level employee X, talking while they are employed with your compet

In general although, how do you deal with this kind of prove a negative situation? Hire high level employee X, talking while they are employed with your competitor or not, how do you prove they don't 'copy files' or bring documents with them to their house? Search their house? How do you know they didn't put them in some secret location?