Yes, public companies disclose this in the Proxy Statements. It is called a Definitive Proxy Statement Form 14A. You can go to www.sec.gov to find these filings. Here is a link to one that was released today.
Definitive Proxy Statement Form 14A
Assuming that most of the executives would have employment agreements or at least, on-competition contracts, most of them include provisions that prohibit working, consulting and advisory role, owning <5% or more of investments in any competitive organization. It usually indicates that any external activities can not detract from their corporate responsibilities. Other prohibitions and ethics policies should preclude any related issues. Given the recent outcry on executive excesses and the need for transparency, we expect there may be new and more encompassing legislation forthcoming.