I did allot of reading and allot of research on all of the boards before I chose. I took into consideration the style of the SP's writings, her posts and website(s). If the website is well done and tasteful it shows that the lady invested some time and money into her business. If she lists in many places, it also shows an investment. Personally, I would never choose from CL or 123...to easy to be taken or scammed. Also I read reviews - even those from old archives. I googled and searched extensively. I tried to contact board members, but when you have no history (as I did not then) it is difficult to get replies.
And then chose based on the facts I could find coupled wit my "gut" feel (mostly this because I am an intuitive decision maker). I corresponded and if I liked the style and content of the replies, I booked. So far, I must say, I have been very lucky / fortunate and I am happy with my choices so far...and particularly very happy with one...