The interrogate RCX command is a very useful way to see what is going on with your program while it is running. Interrogate RCX opens up continuous communication between your computer and the RCX. To open interrogate RCX go to Projects -> Interrogate RCX.
Here is a program that sets up a sensor and a container:
Run the program above and then open up Interrogate RCX. Your RCX must be directly in front of your tower for optimal communication:
As you can see from the screenshot above, it is easy to check on how all of the ports are configured and watch the value of containers.