A few things:

• Knowledge has to be material, eg. significant to company's prospects/fortunes

• All trades by C-level execs, board members have to be reported, date and quantity

• There is a reason that shares are often granted by board at set times in lieu of top level people buying on open market

• Similarly, top execs often have set recurring sales set up for their spending, asset diversification purposes. So a CEO might automatically sell 1000 shares the Friday after each earnings call so it's easy to see that it's a recurring trade and not a sale because something is wrong

• Companies have "Quiet Periods" where employees, execs, board members, etc. CANNOT trade. These are in weeks leading up to quarterly earnings, before a major acquisition, before a major lawsuit concludes, and such.