There is nothing 100% fail safe.  Forget the thing about asking if she is a cop.  They can lie.  You can go to only reputable & well reviewed providers who have a history of an internet presence under one name.  Never discuss sex of any kind, even in code, whether on the telephone or in an email & avoid providers who do.  The money thing really has nothing to do with it.  However, you never want to put the money & sex together, hence you not discussing sex.  Keep in mind that code words can now be legally used in court, because they are considered common knowledge now, rather than just industry lingo, because of online glossaries.