Managers have the ability to change a player's picks after the league's deadline or after the game has occurred, and even for a past week. Every change a manager makes is recorded in the Manager Change Log. This page is accessible to all players in the league via the Settings -> League Settings page, in a link just below the settings shown on the left-hand side. 

Also, when a manager submits picks for a player, you can see this by hovering over the player's picks on the Standings, Player Picks tab.