It is on by default, but you can set it by placing
Option Infer On|Off at the top of the code file. The option will apply to the whole document.
You can switch it on/off via the menu in Visual Studio:
Project > [Project] Properties > Compile Tab > Option infer
On|Off in the drop-down menu. The option will apply to the whole document.
All new projects
You can switch it On by default for all new Projects by selecting:
Tools > Options > Projects and Solutions > VB defaults > Option Infer
On|Off in the drop-down menu.