Buttons most often appear as a glowing red or green spot on a white panel. Sometimes, they appear other ways, such as the white Alarm with a red frame. When pushed, a button will usually change color.
During most of the game, Nick pushes the button. It is different sometimes though. In the case of alarms, Labcoats and Construction Workers push them, and the Nick turns them off. Sometimes, Nick will have to use Mind Control to get someone else to push the button.