How to detect when the Enter Key Pressed with Arbortext Command Language

It depends what you mean by "detect". If you want to execute a specific action when Enter is pressed, you can use the map command to do so, e.g. 

map Enter { response("You pressed enter!"); }

Of course, if it's an edit window, doing this will bork your quick tags feature (the menu of valid tags that can be inserted at the caret location).

If you want to know when Enter is pressed, but still have it carry out its usual effect (like quick tags), you might need to use a Java or Javascript event listener instead.