Instead, the past performance of a person is the best indicator of future performance. Asking a person to perform a job test and demonstrate his skills helps discover the true stars.
As a person interviewing for a job, you can't count on the company to always give you the opportunity to demonstrate your skills. So I suggest you do what many graphic artists do — Start to build a portfolio of your accomplishments. Lead the high-profile projects in your company and then create a visual of your success. This could be a graph of the costs you cut for the next 12 months or the revenue you generated over the next year. You could also draw a Venn diagram to show how the intersection of your skills creates a distinct advantage for your new employer.
And when you're in the interview, go into demo mode at your first opportunity. Treat the interview as a consulting opportunity and show the hiring company how new technologies or a new marketing plan will drive customers to its doorstep.