Here's how Silicon Valley's startup guru can instantly tell if you should start a company

Steve Blank is a legend in Silicon Valley.

With four IPOs and eight startups under his belt, students and budding entrepreneurs frequently seek him out for advice, including what to do with their careers.

But one thing Blank has learned from working in Silicon Valley since 1978 : building a startup or becoming a consultant isn't a hard choice. If you're cut out to be a startup entrepreneur, you already know it.

On-stage at the Startup Grind conference in Redwood City, California, Blank told the audience about the question he gets most often as a professor during the courses he teaches at Stanford, Berkeley and Columbia.

Students often come to him torn between a great job offer at McKinsey or wanting to work on the startup they've been doing with their friends on the weekend, Blank explained.

"My advice is always to take the job," Blank said.

Those who need to ask, he explains, have already made their decision - the startup life is not for them.

"McKinsey is the world's best job. Startup is the world's best calling," Blank said. "A job and a calling are not an equal choice."

Being an entrepreneur is a calling unlike others. You have to be crazy, you have to be willing to work in the shower or wake up in the middle of the night to jot down notes, Blank explains.

If you're not already "all in" on the startup, says Blank, then it's probably not your calling.