Like you stated, "if it's too good to be true", I am always skeptical of pictures that are absolutely flawless in every way, meaning perfect body, skin complexion, hair, make up, etc. I very cautiously approach any ad that has much lower rates than current market or seems to be available 24/7, especially those available for same day or last minute appointments.

Next I would research the contact information, doing multiple searches on multiple review sites. Search each criteria individually as you may find a match to only one of name, phone, or email. I have spotted a couple of known ripoffs that still use the same phone number/email despite changing their ad, possibly going by a new name this time around. You may want to consider posting an ISO message on a local board, someone may backchannel to you, but would not post a public comment or review for numerous reasons. If this is a touring lady visiting your area, you can do the same, post a message to local board of her hometown. Also, getting to know people and networking among your own community, I was able to meet an under the radar lady at a "meet and greet" event held by a couple of local ladies.  

These are somewhat broad generalizations, and only to be used as a guideline, it's just a couple tips that have worked for me.
best regards, mr.man