Two big red flags:
1. Her only reviewer(s) only has one review
2. Her reviewer has a few reviews, but scores girls much higher than her other reviewers
I've known about reviewers who 'barter' for better reviews - they ask for a variety of things including discounts, freebies, and special favors. Those guys can be tough to detect, as they may review lots of girls and give a variety of scores. I've known of several like this...they barter with who they can, but see other girls too and review them more honestly. These men can overlap with the other group known as 'shills'...they also review and otherwise promote girls/agencies in return for special treatment. Even for seasoned hobbyists and board readers, that can be very difficult to detect (altho sometimes it's pretty easy).
My advice would be to find girls with a few reviews written by reviewers with a few reviews. Don't trust the numbers - just use the review text to get a general feel for how she runs her business, and how happy her clients were with the services they received.
Just because a girl has only one review, or her reviewer has only one, doesn't necessarily mean she (and he) are not legit...some girls don't work that much, some clients don't review that much, some reviewers only review when asked to. A very low percentage of dates translate into a review, and some business models don't attract as many people of the reviewing mindset. But if you want to play it cautious, it's better to see a well-reviewed girl.