Like most definitions in this world (including GFE and PSE, for example) "well reviewed" means many things to many people.  For me it means at least 2 or 3 pages of reviews over a couple of years, preferably by well-established, credible reviewers, and with rankings averaging a least 8s.  Others will no doubt differ.

I would go with at least three reviews in the last six months.

I would be satisfied with as few as 5 or 6 reviews if at least 2 or 3 of the reviewers were well known and generally respected members.

With at least 10, you can see if she is consistent in the ratings she gets.  One or two just isn't enough.  The more reviews, the less likely that one bad one will be typical of her service, but rather just a bad day on her part, or no chemistry between her and the client.  Look at who reviewed her, they should have a good amount of reviews they have written.  I'd be wary if most of her reviews are by guys with only 1 or 2 to their credit.

Swim

until confirmed by later reviewers.  I would say 6-10 minimum with at least a couple by experienced reviewers.  I also look for consistancy.  Really need to read the texts as scoring can be so subjective depending on an individual's desires.  
Anyone can get a low or bad review...  each coupling is a unique pairing of 2 unique indivuals & chemistry is not guarenteed...  I can (if otherwise consistant) discount 1 low review per page of reviews.   Avoid repeated wide swings (again confirmned by the texts)...  which indicates to me she is moody or does not connect with some who are not her type.  
H

Also, If the lady participates in discussions on her regional board, and her posts are well met, that's an even stronger indicator that you've picked a winner.

Since I began reading reviews I have learned something about reviews and YMMV [your mileage may vary]. Sometimes a bad review is true. I read one recently about a young lady who appeared on P411. I was initially interested in her, but never called because she got some bad reviews. In one of the bad reviews, I read that there was a second woman in the room who was not part of the date. That would have made me uncomfortable too. She probably did it because she was new to the business and did not feel safe alone, but that is just one example of the type of bad review I read, which made me not want to book an appointment.

Other bad reviews of providers that come to mind have stated the picture posted was not who they met once arriving at the room. This can be a bait and switch, or just that the the images were very old and the lady gained a lot of weight, or cut her hair super short while the pictures showed her with pretty long hair etc.

However, I have found some women get bad reviews because YMMV. I just booked an appointment with a new provider for this week. I have not seen her before. She has many great reviews and some bad reviews. I read them all and came to the conclusion that the bad reviews were a YMMV situation. When I clicked on the screen names of those who submitted the bad reviews, I noticed that many or all reviews they posted was a bad review, often never scoring the lady over a 6 or 7. IMO, these guys may be physically unattractive, don't not groom well before the date, or may be asking the ladies for things they don't feel comfortable providing.

Just because its a sure thing, does not mean you should not shave and shower thoroughly before a date. You should always wear decent clothing to a date and read the ladies web pages [services and etiquette] and read her reviews, to see if she offers what you want. Just because one lady does not offer BBBJ, does not mean she is a bad provider. She just may not be the right one for you.

I have always had great experiences with my providers, because I am always showered, I smell good, I leave the gifts in plain site exactly the way she asks for on her website and never expect services that are not listed on the verification site or her webpage.

most of her reviews are written by guys who only have 1 or 2 reviews, it could be LE.  The reviewers should have a good number of reviews spread over time; meaning at least 6-8 reviews over a one year or longer period.