I don't.
I usually  keep it short and sweet. The only thing I ask if I don't see it in the ad or after searching other ads or reviews is the rate.  And even it is in other ads or reviews I prefermto confirm  I just won't put myself in a situation without knowing the rate upfront.  
The rate I'm asking is for companionship or possibly a massage.  That's all.  As for specific services I rely on reviews or posts to boards but I never talk about it on the phone nor in texts even if they ask me.  Primary reason is IT GOES WITHOUT SAYING that neither I nor you nor anyone else here would ever pay for illegal services nor would I want anyone with a record of me stating that I am.  Second the phone call is usually the first contact of building repore and mutual trust and respect therefore I wouldn't want her to do anything illegal either. Last I don't think it does that much good.  The best thing I can do is follow the standard rules and mores that I've learned over the years. including not wasting their time, showing them respect and always maintaining the highest levels of hygiene and dress.
After all of that I enter each new encounter with the acceptance that there are no guarentees and Iz Paz my monez and I take my chances. Sometimes I leave bummed out and have to hope for better next time.  BUT also there are those few cherished unexpected gems that rock my world and keep me coming back.  One of those moments occurred earlier this week.  I saw Jenna in North Hollywood. It was one of those times that all the stars aligned.
Hope that helps.