How can you tell if a freelancer is good at what they do?

[Julzwriter]

I recently started hiring freelancers to help
me with some tasks on my website. I had gotten
good services from various freelancers until today, when I got
very poor work from a freelancer. The surprising thing was
that she had a number of positive reviews on her profile
from a number of previous buyers. I was not expecting
such mediocre work based on her rating. It made me think
that the maybe the reviews she had were fake.
Has this ever happened to anyone else here? You hire
a freelancer who is highly rated and then they deliver poor
quality work?