As a film/DVD reviewer, there's a couple of things I do to let the readers know about my personal tastes/biases that could also apply to SP reviews:
1. Start the review with a couple of lines of Likes and Dislikes. For example, this could translate in the SP context as:
Likes: Petite women, brunettes, girl-next-door, touching, kissing, sensuality, eroticism, stimulating conversation.
Dislikes: Fake breasts, tatoos, biker/stripper look, smokers, hardcore sex.
The reader should quickly deduce from the above that the reviewer is more into total GFE experiences than hardcore PSE stuff, and that he really doesn't care about anal sex, deep throat, CIM, MSOG, etc...
2. Add comparisons in your conclusion as points of reference. For example:
"Offers services comparable to sp_name1, sp_name2, sp_name3"
It's difficult to write objective film reviews because you need to be able to dissociate yourself from your own personal tastes ("Like" and "Good", "Hate" and "Bad" are not the same thing...) Now when it comes to SP's, writing objective reviews is darn near impossible because:
1- We are dealing with human beings.
2- We are interacting with said human beings in an intimate fashion.
3. We are men. And men being men, they will always "brag" about their sexual prowess.
But the couple of suggestions above should help the reader "filter" the truth from the embellishments.
Oh, and reviews? Read more than one, and as someone else suggested, try to find reviewers who's tastes mirror your own.