Very hard. You run into all types of candidates who just aren't there yet: people working on research that's irrelevant to real world applications, people who have done data analysis/BI work that brand themselves as "data scientists," those who have the pedigree but cannot process and explore real-world data, those who have good analytical chops but not the distributed or advanced modeling experience, etc.
I've witnessed it first-hand, and it's tough to find the right person.