Start working with them. It becomes immediately apparent. I'm on my second startup, we just raised funds and brought on a VP of marketing, and we did it by telling one guy "congrats you're on the team" and a week later realized that he wasn't the right fit, so told him "goodbye" and then we told another guy "congrats you're in the team" and after a week we all agreed he is totally awesome, and he's still the guy, and we wouldn't have been looking for him if we didn't think of the other guys first week as his job interview.
Also, sales & marketing != biz management