A productive Forth system, for me, has stack effects commented into each of the words, in Thinking Forth style has a literate style which flows, has plenty of testing words which help you build up mock objects that you can test your words on, and often real tests which tests each word to make sure it does what it's supposed to (e.g. place a file id on the stack, run your word, and then test the file id to make sure it has the contents that you expect.) Developing and testing should happen often and frequently in the REPL; it's a very dynamic process. This style is very different from ALGOL or Lisp style structured programming where it can be annoying to test things (though in Common Lisp you should just be able to jump into a running function and see what's going wrong), but in its stead the compiler gives you a lot of assurance as to what's going on.