Pasha explained everything correctly. If you have two computers connected via a local network, it's enough to share the folder with the Heroes save files and calmly observe the situation on the second computer during the opponent's turn; this is not visible at all in the broadcast. The only way to avoid the possibility of cheating is to play from a remote server, where the game itself is located outside the players' computers, and they simply have no way to obtain any information (the saves and the generated map are stored on the server, and access to it can be easily restricted).