Good writing is of course the first answer to that.
But there is a lot of good writing out there. How do you assess whether a writer will be a good fit or has what it takes to survive in this crazy Publication World.
Well, you pay attention to the little things they tell you.
I've been in touch with a prospective author for about three years now, off and on. Recently she sent me her full manuscript. We've had back and forth emails about it, and today very casually she mentions this:
My laptop died two months ago, so 50% of what you are hopefully reading was written on my iPhone.
Yea, that. That's dedication. That's perseverance. THAT'S what I look for.