For me, it snuck up on me in my thirties. I lucked into school for design, and.... found my home. Never had a thing where I "identified" as the job.
How do I know? Hard to say... it just feels incredibly clear. It feels right like no other work ever has. It's sorta the old canard about "loving what you do."
In other words, I know because I love the work. I lose myself. I can spend whole days working, not drinking, not eating, just working. Hard to ignore.