The external metric that one can use is: "did it make my code and my ability to explain things less complicated and more understandable to other people". If ye

The external metric that one can use is: "did it make my code and my ability to explain things less complicated and more understandable to other people". If yes - that person was ready. If no, if as a result both code and the language of that person got filled with formal and obscure complications... then no. Not ready.