This one I found an interesting way.  By studying the typing style.  The person in question is "Ariana" on Craigslist (http://sandiego.craigslist.org/ers/80288893.html)

The guy who reviewed her has the identical typing style as she does in her craigslist posts.  This means the reviewer is probably her.  His review of Precious and one of Arianas posts are almost identical  Here they are side by side.  Note the use of all caps and more specifically - the exact same errors related to commas.

His Review of Precious (diff girl):
"OKAY PRECIOUS HAD SOME RULES ,BUT I TOTALLY RESPECT THAT FROM A SOMEWHAT CLASSY GIRL. SO SHE ASK ME HOW I LIKE IT,AND I ASK HER HOW ABOUT AN NO CONDOM BJ. WELL I PASS ON THAT ONE ,BECAUSE..."

Both people consistently put a space before the comma like this ,or puts no space at all between comma and words like,this.  

Arianas Post on craigslist.  Again note the commas:
"HELLO FELLAS I'M ARIANA ,AND I'M FEELING VERY FREAKY TODAY AND I'M WANTING TO SASTIFY ALL GENEROUS GENTLEMAN. I'M NEW ON CL AND DONT HAVE ANY REVIEWS YET,BUT I HAVE CLIENTS..."

Very distinct typing errors and clearly this reviewer is fake Ariana herself, or the person who posts her craigslist ads.