It's clear when you get it. It's clear when you're talented. And you know how someone can tell? By looking at your book.
The three things you need in this industry is a good portfolio, a good personality and good connections.
I've written about it before *ahem*
What exactly do you mean by "portfolio"?
What your portfolio says about you
Beef up your book
What not to put in your book aka Don't Columbus other people's work
Don't Benetton Your Book
Your book will never be finished
but here is even more great advice for copywriters, art directors and developers on what to put in their book.
What Not To Put In Your Book
From Big Orange Slide