Situation: I have upgraded two Nvidia Optimus laptops from Linux Mint 17.3 to version 18.
Problem: After upgrade, and a fix for Nvidia drivers, I miss the applet in system tray indicating, which graphics card is currently in use.
After a while, I found out, it is a separate package called
sudo apt-get install nvidia-prime-applet
and enabling it in Applets (one of Mint's configuration application).