How to determine if installation is succesful?

[xsintill]

I want to perform a CustomAction at the very end of a succesful installation. I've got the feeling this is very simple and i am just looking over things.:o
How do you determine if installation is succesful and use that conditionally for executing the customaction?

[RobertDickau]

Perhaps create a "commit" custom action, which runs only if all the system changes made during deferred mode succeeded.