Another test you can use is: always produce. For example, if you have a day job you don’t take seriously because you plan to be a
novelist, are you producing? Are you writing pages of fiction, however bad? As long as you’re producing, you’ll know you’re not merely using the hazy vision of the grand novel you plan to write one day as an opiate. The view of it will be obstructed by the all too palpably flawed one you’re actually writing.
I appreciate his acknowledgement that it’s ok not to know what you want to do when you’re young. So many people see changing your mind, or trying something different out of curiousity, as failure.