[WiX-users] Check for Installed app

Can you give me the code to check for a previously installed version of my app? Then, prevent install if it is there? Currently, I am allowed to install the same app twice and I want to prevent that. Also, do you have example code on how to check for an old version installed? Then,

[WiX-users] Run external Exe on Install

I have a prerequisite that must be run before my installer continues. Can you give me a code example on: 1. How to call the executable. Custom action? Script? 2. How to verify that it ran. Look for a file in a directory? 3. How to pause the install until it is verified.