You can search the review and verification sites for a particular provider's history. While this is no guarantee (since LE can fake it), it does provide a little more assurance than just blindly responding to a backpage ad. Some sites to try are: www.preferred411.com, www.datcheck.com, www.theeroticreview.com as well as this site.
Use a search engine for their name, their number... Check p411 as Cobra suggests... Use a pic verification site... If it's an OOT area code, find out what city and check ECCIE and BP ads from that city... etc... etc... etc...